We investigated the relation of drug use and sexual behaviour to hepatitis C virus (HCV) infection among 80 intravenous drug users (IVDU) attending a methadone treatment program in Rome. Antibodies to HCV (anti-HCV) were found in 54/80 of IVDU (67.5%). Presence of anti-HCV was associated with duration of intravenous drug use and frequency of needle sharing (p less than 0.003 and p = 0.02, respectively, by chi-square for trend). No association was found between sexual behaviour and anti-HCV prevalence.