Secondary service connection under 38 CFR § 3.310 covers conditions caused by or aggravated by an already service-connected condition. The condition does not need to have arisen during service — only to be medically connected to a primary that did. Under Allen v. Brown(Fed. Cir. 1995), aggravation of a non-service-connected condition by a service-connected one also qualifies. Three elements: existing primary SC, current diagnosis of secondary, medical nexus at the “at least as likely as not” (≥50%) threshold.
Under 38 CFR § 3.310, a disability is secondary to a service-connected condition if it was either caused by or aggravated by that already service-connected condition. Separate pathway from direct service connection — which requires showing the condition was caused by something that happened during military service.
The leading caselaw is Allen v. Brown (Fed. Cir. 1995). Allen established that aggravation of a non-service-connected condition by a service-connected condition also qualifies for secondary service connection. Meaning: even if the secondary existed before — if the service-connected primary made it measurably worse, it can still qualify.
Example: a veteran is service-connected for a left knee injury. The knee injury caused an altered gait, which in turn caused a right hip condition. The hip didn't happen during service — but it was caused by a condition that did. The hip qualifies as secondary to the left knee under § 3.310.
1. An existing service-connected disability. You must already have at least one condition the VA has granted service connection for — the “primary.”
2. A current diagnosis of the secondary condition. A medical diagnosis confirming the secondary exists. Symptoms alone are insufficient — the VA will not grant service connection without a diagnosed disability.
3. A medical nexus linking the two. Medical evidence — typically a doctor's opinion or IMO — stating the secondary is “at least as likely as not” (≥50%) caused by or aggravated by the primary service-connected condition.
The service-connected condition directly caused the secondary. Example: a veteran with a service-connected back injury develops radiculopathy (nerve pain shooting down the legs) as a result of spinal damage. The radiculopathy would not exist without the back condition.
The service-connected condition made a pre-existing non-service-connected condition permanently worse beyond natural progression. The VA must determine the baseline severity of the secondary condition before the aggravation occurred. The veteran is then compensated for the degree of worsening above that baseline.
Example: a veteran has mild sleep apnea predating service connection. After being rated for PTSD, the sleep apnea worsens significantly — PTSD-related hyperarousal and weight gain from medication make it measurably worse. The veteran is compensated for the aggravation above the pre-existing baseline.
1. On VA Form 21-526EZ, clearly state the claimed condition is secondary to the existing service-connected condition. Example: “Right hip osteoarthritis — secondary to service-connected left knee patellofemoral syndrome.”
2. Submit a nexus letter. Strongest evidence available. The letter states the provider's qualifications, reviews medical history, explains the medical connection, and concludes with the “at least as likely as not” opinion language.
3. Include supporting medical records showing progression of the secondary condition and any documentation linking it to the primary.
4. Write a personal statement describing how the secondary developed and how it affects daily life — under 38 USC § 1154(a) lay testimony carries legal weight.
No nexus opinion. Relying on the C&P examiner alone is risky — many provide negative nexus opinions because the question was not framed correctly or the connection was not clearly documented.
Claiming the wrong primary. Make sure the primary you link to is the one most medically connected to the secondary.
Weak nexus language. “May be” or “could be” is speculative — fails the “at least as likely as not” threshold under 38 CFR § 3.102.
Not documenting aggravation baseline. If claiming aggravation rather than causation, evidence showing the secondary's severity before the aggravation began is required to calculate the compensable degree of worsening.
Under 38 CFR § 3.310, a disability is secondary if it was caused by — or aggravated by — an already service-connected condition. This is a separate pathway from direct service connection; the secondary condition does not need to have arisen during service, only to be caused or made worse by a service-connected primary.
Allen v. Brown (Fed. Cir. 1995) established that AGGRAVATION of a non-service-connected condition by a service-connected one qualifies for secondary service connection. The VA compensates for the degree of worsening above the baseline severity that existed before aggravation began.
Three elements: (1) an existing service-connected disability (the "primary"); (2) a current diagnosis of the secondary condition; (3) a medical nexus linking the two — typically a doctor's or IMO opinion stating that the secondary is "at least as likely as not" caused by or aggravated by the primary, per 38 CFR § 3.102.
Proximate causation: the service-connected condition directly caused the secondary (lumbar spine injury caused radiculopathy). Aggravation: the service-connected condition made a pre-existing non-service-connected condition permanently worse beyond natural progression (PTSD hyperarousal aggravated pre-existing mild sleep apnea).
Sleep apnea secondary to PTSD (weight gain + hyperarousal). Radiculopathy secondary to lumbar or cervical spine. Hip / opposite-knee secondary to a service-connected knee (altered gait, compensatory overuse). Peripheral neuropathy secondary to diabetes mellitus type 2. Erectile dysfunction secondary to PTSD medications. Hypertension secondary to sleep apnea or PTSD.
Four common reasons: no nexus opinion (relying on a C&P examiner is risky); claiming the wrong primary (pick the most medically-connected one); weak nexus language ("may be" or "could be" instead of "at least as likely as not"); and failing to document the aggravation baseline when claiming aggravation.