As there is insufficient data available on the spread of hepatitis C virus (HCV) infection by sexual transmission, the prevalence of anti-HCV was determined in several sexually promiscuous groups (78 female prostitutes, 55 clients of prostitutes and 146 homosexual men) who denied intravenous drug abuse. The overall HCV infection rate was much higher in the sexually promiscuous groups (8.97%, 16.36% and 5.48% respectively) than in voluntary blood donors (0.48%), suggesting that HCV infection can be transmitted by sexual intercourse. Consequently, promiscuity may be a risk factor for this infection.