While hepatitis C virus (HCV) is known to be transmitted parenterally, the role of sexual transmission remains unclear. In order to examine the association of sexual risk factors with HCV seroprevalence at a time when unprotected sexual practices were still quite common, 435 homosexual men recruited from a municipal sexually transmitted disease clinic with behavioural data and serologic specimens from 1983-1984 were evaluated. Overall, 25% of men reporting injecting drug use (IDU) and 5% of men with no IDU were anti-HCV positive; the rate in the non-IDU was significantly higher than age-matched rates in blood donors (summary odds ratio 3.5, 95% confidence interval 2.8-4.2). In addition to IDU, amphetamine and phencyclidine use were also associated with anti-HCV positivity on univariate analysis. Sexual risk factors for anti-HCV positivity included anal receptive intercourse, 'fisting', having an IDU sexual partner, a self-reported history of genital herpes and HIV seropositivity. On multivariate analysis, only IDU was significantly associated with anti-HCV positivity. Thus, sexual practices appear to play a minor role in transmission of HCV.