The epidemological and clinical features of hepatitis C virus infection have been evaluated in a cohort of 227 intravenous drug users enrolled at a drug dependence treatment center in the Veneto area in 1992-1993 and followed periodically. Hepatitis C virus infection was detected using second-generation anti-HCV ELISA in 171 (75%) subjects at enrollment. Anti-HCV seropositivity correlated with: a) the duration of drug abuse: 91% of intravenous drug users injecting for more than 8 years were seropositive as compared to 40% of those with a history of abuse lasting 4 years or less, p < 0.001; b) sharing of injection equipment: 85% anti-HCV positive intravenous drug users had shared at some time as compared to 64% seronegative subjects, p < 0.001; c) seropositivity for immunodeficiency virus infection: 25% anti-HCV positive intravenous drug users were coinfected as compared to 3.5% anti-HCV negative, p < 0.001; d) markers of ongoing (two cases) or previous hepatitis B virus infection were detected in 62% of anti-HCV positive but in 21% of anti-HCV negative cases, p < 0.01. Two initially anti-HCV negative intravenous drug users seroconverted during follow up giving an incidence rate of hepatitis C virus infection of 6.2 per 100 person-years. During the survey abnormal alanine aminotransferase levels were detected in 75% anti-HCV positive but in 24% anti-HCV negative cases (p < 0.001), with significantly higher levels in the former. These findings suggest that the circulation of hepatitis C virus among intravenous drug users has been decreasing in recent years, although new infections still occur.(ABSTRACT TRUNCATED AT 250 WORDS)