The article describes drug use behaviors, AIDS knowledge, risks for HIV infection, and HIV seroprevalence in drug users entering rehabilitation in Ho Chi Minh City, Vietnam. A cross-sectional, descriptive survey of all new entrants to a residential drug treatment center was conducted with linked HIV serology between July 1 and July 14, 1995. A total of 105 subjects participated: 101 men and 4 women. HIV serostatus was available for 88 subjects. Forty-seven percent (41 of 88 subjects) were HIV-positive. Median age of the subjects was 38 years. Mean length of injection drug use was 13.2 years (range, 1-27 years). The primary drug of injection was opium (96%), although 59% of subjects also injected "Western" drugs such as sedatives or tranquilizers. Eighty-two percent (86 of 105 subjects) correctly answered at least 7 of 10 AIDS knowledge questions, and only 28% (27 of 97 subjects) reported any needle sharing in the last 5 years. Seropositivity was associated with a history of previous treatment for drug abuse (p = 0.002), longer history of injecting drugs (p = 0.003), use of Western drugs (p = 0.03), and higher educational level (p = 0.05). Multivariate analysis found that the independent predictors of HIV seropositivity were history of previous treatment for drug abuse (p = 0.06) and longer history of injecting drugs (p = 0.05). Despite low levels of self-reported needle sharing and high levels of AIDS knowledge, HIV seroprevalence was high in this sample. The potential for epidemic spread of HIV in Vietnamese drug users is substantial. Risk-reduction programs and intense AIDS education projects targeting the population of drug users are necessary to control the AIDS epidemic in Vietnam.