Although depressed individuals commonly report decreased libido, it was not known if such changes are accompanied by neurophysiological alterations. Preliminary studies suggest that some depressed men may manifest diminished nocturnal penile tumescence (NPT), an objective measure of erectile capacity. We report NPT findings in 34 male outpatients with major depression (SADS/RDC) and an age-matched group of 28 healthy controls. A 3-night electroencephalographic (EEG) sleep/NPT protocol was utilized, with penile rigidity (buckling force) determined on night 3. Analysis of night 2 data by MAN-COVA revealed significant effects for age, the covariate (F = 2.86, p = 0.002), and diagnosis (F = 2.32, p = 0.02). Depressed men had significantly diminished NPT time (F = 16.8, p less than 0.001), even when adjusted for sleep time (F = 13.4, p less than 0.001) or rapid eye movement (REM) time (F = 7.2, p less than 0.01). NPT time was reduced by greater than or equal to 1 SD below the control mean in 40% of depressives and was comparable to the level seen in 14 nondepressed patients with a clinical diagnosis of organic impotence. An intermediate proportion of depressed patients (38%) had maximum buckling forces less than or equal to 500 g, indicating diminished penile rigidity, when compared to controls (16%) and men with presumed organic impairment (93%) (p less than 0.001). Diminished NPT time and low buckling force were associated with a history of erectile dysfunction within the index depressive episode (p less than 0.001). These findings suggest that depression in men is associated with a potentially reversible decrease in erectile capacity, which may be associated with significant sexual dysfunction.