In the present study data on the incidence of HBV and HCV were used to indicate the prevalence of and trends in risk behavior, assuming that drug users (DUs) who become infected with HBV or HCV are also at risk for infection with HIV. In addition, we determined to that extent the transmission patterns of HIV, HBV and HCV differed. DUs were selected from a cohort study in Amsterdam, had at least one follow-up visit between December 1985 and September 1989 and reported never to have had homosexual contacts. Among 305 DUs, of whom 70% injected recently, the prevalence of HIV, HBV and HCV were 31%, 68% and 65% respectively. These prevalences were strongly interrelated and the same risk factors were found. The cumulative incidence of either HIV or HBV or HCV was 30% among prevalent HIV-negatives. Despite a previously reported reduction in risk behavior, only the HIV incidence tended to decrease initially, and after 1986 the incidences of HIV, HBV and HCV remained disturbingly high and stable (mean: 4, 9 and 10 per 100 person-years, respectively). As at present HBV appears to be transmitted more heterosexually than HIV in our study group and the HIV-epidemic may follow the HBV-epidemic in its transmission patterns, preventive activities targeted at both injecting and sexual behavior should be expanded.