Several clinical studies suggest that individuals with posttraumatic stress disorder (PTSD) experience neuroendocrine system alterations, resulting in significantly lower plasma cortisol. To test this hypothesis, morning serum cortisol was compared among a national sample of Vietnam "theater" veterans (n = 2,490) and a sample of Vietnam "era" veterans (n = 1,972) without service in Vietnam. Analysis of covariance was used to compare cortisol concentrations after adjusting for 9 covariates (education, income, race, age, smoking status, alcohol use, illicit drug use, medication use, and body mass index). Adjusted cortisol was lower among theater veterans with current PTSD but not era or theater veterans with lifetime PTSD. Among theater veterans, cortisol was inversely related to combat exposure, with veterans exposed to heavy combat having the lowest concentrations. Analysis of plasma cortisol, together with other clinical data, may be instrumental in the future diagnosis and treatment of stress disorders.