What "service-connected" means
A disability is service-connected when VA concludes, on the record, that a current condition is linked to the veteran's qualifying period of active service. The legal framework lives in [src], which sets out the principles of service connection, and is supplemented by sections covering aggravation, presumptive conditions, secondary conditions, and treatment-related disability. A grant of service connection unlocks a rating under [src], and that rating is what produces the monthly compensation amount.
Most claim decisions turn on whether each of the three elements below is supported by evidence VA can verify. Understanding the test, and the theories that change which elements need to be proven, helps a veteran or representative gather the right records before filing.
The three-element test for direct service connection
Under [src], a direct claim must establish three elements. Federal Circuit decisions, including Shedden v. Principi, restate the same framework: each element has to be supported by competent evidence before VA can grant service connection on a direct theory.
- A current diagnosed disability. VA needs evidence that the condition exists today, not only that it once existed. Acceptable proof typically includes treatment notes from a VA or private clinician, imaging, lab results, or the C&P exam report. A condition that fully resolved before the claim period usually cannot support service connection on its own.
- An in-service event, injury, or illness.This is the "something happened in service" element. Proof can come from service treatment records (STRs), personnel records (DD Form 214, deployment orders, performance evaluations), unit histories, or lay statements describing what the veteran or witnesses saw, felt, or did. The event does not need to be dramatic; a single documented complaint of low-back pain in an STR can satisfy element two.
- A medical nexus connecting (i) and (ii).A clinician must opine that the current disability is caused or aggravated by the in-service event. The standard phrasing VA looks for is that the condition is "at least as likely as not" related to service. The opinion can come from a C&P examiner, a VA treating provider, or a private clinician who has reviewed the relevant records.
The five theories of service connection
"Direct" is only one of the recognized theories. A claim can be granted on any theory the evidence supports, and a single condition may be filed under more than one theory in the alternative.
1. Direct service connection (§ 3.303)
The standard three-element framework above. Most first-time claims start here. [src]
2. Presumptive service connection (§§ 3.307, 3.309 and the PACT Act)
For certain exposures and certain conditions, VA presumes the nexus element so that veterans do not have to prove it [src]. If a veteran served in a covered location during a covered period and is diagnosed with a listed condition, VA may grant the claim without an in-service-onset record. Examples include Agent Orange exposure for Vietnam-era veterans, burn-pit exposure for post-9/11 veterans expanded under the PACT Act, contaminated drinking water at Camp Lejeune, ionizing-radiation exposure for atomic veterans, and Persian Gulf undiagnosed-illness presumptions. Our pages on the PACT Act, Agent Orange, and burn pits walk through the eligible locations and listed conditions for each program.
3. Secondary service connection (§ 3.310)
A condition that is not itself directly tied to service can still be service-connected if it is caused or aggravated by a condition that already is [src]. Common examples include a contralateral knee condition that develops from an altered gait caused by a service-connected knee, hypertension or sleep apnea linked to service-connected PTSD, or radiculopathy that develops from a service-connected lumbar spine condition. Our secondary conditions reference lists the most commonly granted pairings and the medical literature that supports them.
4. Aggravation of a pre-service condition (§ 3.306)
If a veteran entered service with a pre-existing condition that was made permanently worse beyond its natural progression, VA may grant service connection for the aggravation [src]. The presumption of soundness applies: if the entrance exam was normal, VA generally presumes the veteran was sound at induction unless clear and unmistakable evidence shows otherwise.
5. Disability caused by VA treatment (§ 1151)
A separate path, codified at [src], allows compensation when a veteran suffers an additional disability caused by VA medical or surgical treatment, an exam, or participation in a VA vocational rehabilitation program. The additional disability is treated "as if" service-connected for compensation purposes when the statutory criteria are met.
The "at least as likely as not" standard
VA medical opinions are written against a probability scale rather than a beyond-a-reasonable-doubt standard. The threshold for a favorable nexus opinion is "at least as likely as not" — a 50 percent or better probability that the current disability is related to service. Anything weaker ("less likely than not," "cannot be determined without resort to speculation") does not satisfy the nexus element on its own.
When the evidence is in approximate balance, the benefit-of-the-doubt rule applies and the tie goes to the veteran [src]. That rule is one of the most important features of the VA-claims system; the burden of proof is preponderance, not certainty, and a 50/50 record is supposed to be resolved in the claimant's favor.
Nexus letters: independent medical opinions
A nexus letter is a written opinion from a private clinician stating, in the appropriate phrasing, whether the veteran's current condition is at least as likely as not related to service. Nexus letters are most useful when the C&P examiner's opinion is unfavorable, equivocal, or based on an incomplete review of the record. They are not required for every claim — a strong C&P opinion alone is often enough — but they can rebut a weak examination.
What raters generally look for in a nexus letter:
- Clear opinion phrasingusing the "at least as likely as not" standard, not vague language like "could be related."
- Documented review of records— service treatment records, post-service medical records, and any prior C&P examination reports. An opinion that contradicts an exam without engaging with that exam tends to be discounted.
- Citation to medical literature when the link is not self-evident, especially for secondary-condition theories where a published mechanism strengthens the opinion.
- Provider qualifications— a specialist's opinion in their area of expertise generally carries more weight than a generalist's.
- Reasoned rationale— the clinician should explain the "why," not just state a conclusion.
Lay evidence: what veterans and witnesses can say
Veterans, fellow service members, and family members are competent to describe symptoms they personally experienced or observed. [src] defines competent lay evidence, and the Federal Circuit's decision in Jandreau v. Nicholson, 492 F.3d 1372 (Fed. Cir. 2007), confirmed that lay witnesses are competent to describe observable symptoms — pain, ringing in the ears, sleep disturbance, mood changes — even when they cannot diagnose an underlying disease.
Lay evidence is especially useful when service treatment records are silent on a condition the veteran experienced. Many service-era injuries, mental-health symptoms, and exposures were never written down at the time. A buddy statement from a fellow service member describing the same incident, or a spouse's statement describing post-deployment changes, can fill the documentation gap.
Our buddy statement coach walks through the elements raters look for: who the witness is, how they observed the events, what they saw or experienced firsthand, and dates anchored to verifiable points (deployments, duty stations, training cycles).
Documenting the in-service event: pitfalls and relaxed standards
The in-service-event element is where many claims stall, because STRs do not always reflect what happened. Two statutory rules relax the documentation burden in specific contexts.
Combat veterans: 38 USC § 1154(b)
For injuries or diseases alleged to have been incurred in combat, [src] directs VA to accept satisfactory lay or other evidence as sufficient proof if it is consistent with the circumstances of the veteran's service, even without official records. The relaxed standard applies to combat-related events; the nexus and current-disability elements still need their usual support.
MST cases: § 3.304(f)(5) markers
For PTSD and related conditions based on military sexual trauma, [src] allows alternative evidence — "markers" — to corroborate the stressor when official reports are absent. Markers can include requests for transfer, sudden performance changes, mental-health treatment, pregnancy or sexually transmitted infection testing, economic or social-behavior changes, or statements from people the veteran told contemporaneously.
What service-connected status produces
Once a condition is service-connected, VA assigns a percentage rating under [src] using the diagnostic code that fits the condition. Ratings run in ten-percent increments from 0 to 100. When more than one condition is service-connected, the ratings combine using the whole-person formula at [src], not simple addition. Our combined rating calculator walks through the math and shows how the bilateral factor and rounding rules apply.
Service-connected status also opens the door to other benefits downstream — VA health care priority groups, dependent benefits at higher rating levels, vocational rehabilitation, and special monthly compensation when the criteria are met. The rating decision letter is the document that controls those downstream paths; keep a copy.
Where this fits in the claims process
Service connection is decided at the rating-decision stage, after VA has gathered evidence and (usually) scheduled a C&P exam. If a claim is denied for lack of one of the three elements, the rating letter will say which element failed. That tells you what new evidence is needed — a current diagnosis, additional in-service documentation, or a stronger nexus opinion — before filing a supplemental claim or higher-level review under the Appeals Modernization Act.
Veterans Benefits Navigator does not file claims, provide legal advice, or represent veterans before VA (38 U.S.C. § 5904). For filing assistance, consider a VSO, CVSO, or VA-accredited attorney. The VA's accreditation directory lists every accredited representative and is searchable by state.