The objective of the study was to determine whether measures taken to prevent human immunodeficiency virus (HIV) infection also lead to a reduction in the prevalence of hepatitis C virus (HCV) infection among intravenous drug users (IDU). Antibodies to HCV, HIV and hepatitis B virus (HBV) were determined in IDU who voluntarily attended AIDS prevention and information centres for the first time between 1990 and 1996. Of the 5473 IDU studied, determination of HCV was done in 3238 cases. The prevalence of antibodies to HCV was 85%. During the first period studied (1990-1992), the prevalence of antibodies to HCV was 84.5%, during the second (1993-1994) 84.1% and during the third (1995-1996) 87%; in the case of HBV the prevalence during the three periods was 74.5, 67.6 and 66.8% respectively, and for HIV it was 41.9, 38.8 and 36.6% respectively (RR: 0.72; 95% confidence interval (CI): 0.65-0.81). Among drug users addicted for less than 2 years, the trend of the prevalence of antibodies to HCV and HBV remained constant, while the prevalence of HIV infection decreased (RR: 0.61; 95% CI: 0.42-0.89). Measures to prevent transmission of HIV in drug users do not lead to a reduction in the prevalence of HCV infection. Further study is necessary to obtain a better understanding of how HCV is transmitted among drug users in order to apply measures which are effective in preventing HCV infection.