As part of an epidemiologic study of the health status of a randomly selected group of Vietnam veterans, we measured the semen characteristics of 324 Vietnam veterans and compared them with a similar group of 247 veterans who did not serve in Vietnam. The participation rate was 81% in both groups. Measurements of sperm concentration, movement characteristics, and head dimensions were performed, using the Cellsoft computer-assisted semen analysis system. We found that Vietnam veterans had significantly (p less than 0.05) lowered mean sperm concentrations (64.8 x 10(6) sperm/mL for Vietnam veterans vs 79.8 x 10(6) sperm/mL for non-Vietnam veterans), and Vietnam veterans were twice as likely to have sperm concentrations less than or equal to 20 million/mL (odds ratio = 2.7, 95% confidence interval = 1.3-5.7). Vietnam veterans also had a significantly lowered mean proportion of morphologically normal sperm heads (57.9% vs 60.8%), with significantly longer mean major axis length and head circumference. The proportion of motile cells, velocity, linearity, amplitude of lateral head displacement, and beat frequency were not different between the two groups. Despite differences in sperm characteristics, Vietnam and non-Vietnam veterans reported fathering similar numbers of children.