The seroprevalences of and risk factors for hepatitis C virus (HCV), hepatitis B virus (HBV), human immunodeficiency virus (HIV), and syphilis were determined among 1257 consecutive non-intravenous drug-using patients attending Baltimore sexually transmitted diseases clinics: 122 (9.7%) had antibodies to HCV (anti-HCV), 192 (15.3%) had antibodies to HBV (anti-HBc), 44 (3.5%) had antibodies to HIV (anti-HIV), and 60 (4.8%) were seropositive for syphilis. For males, independent predictors of anti-HCV were age > 29 years and lack of condom use in the month before their visit. Males with any one serologic marker (anti-HCV, anti-HBc, anti-HIV, or syphilis) were more likely to have each of the other markers. For females, anti-HCV was independently associated with age > 29 years and > 1 male sex partner in the prior month. Females with anti-HIV or anti-HBc were more likely to have anti-HCV. These observations among non-intravenous drug-using patients suggest that sexual transmission of HCV may occur.