A survey was conducted in Vietnam, in November 1969, on the illicit drug experiences of 2,547 army personnel, half of whom were entering that country (incoming) and half leaving (outgoing). Respondents were divided into several groups by age and rank and analyzed separately. Sizable increases were noted in the reported use of most drugs surveyed when compared to previous research. Some major findings were: the majority of marihuana smokers in Vietnam began in the first three months of their tour of duty; heavy and habitual marihuana smokers tended to estimate greater drug use in their units than did casual smokers and nonsmokers; field grade officers were very poor estimators of drug use in their units; marihuana smoking and extent of combat exposure were slightly correlated; 27% of the outgoing enlisted men intended to continue smoking marihuana.