The effect of military service in Vietnam on drug use among veterans is examined on the basis of data obtained from a nationwide random sample of 2,510 young men. It is generally believed that illicit drug use was more extensive in Vietnam in the 1970s than in earlier years of the conflict. Therefore variables were constructed to reflect location of service (United States; overseas, but not Vietnam; and Vietnam) and the time of service (before 1970; 1970 or later). The results of the analysis indicate that: (1) military service in Vietnam had no significant effect on marijuana use while on active duty, (2) service in 1970 or later had a significant positive effect on during service use while in the service, and (3) location of military service and time of service had a signficant interactive effect on rates of marijuana use after discharge from military service. The interaction of service in Vietnam and military service after 1970 has the strongest effect on the veteran's marijuana use in civilian life.