The seroprevalence of hepatitis B virus (HBV) and hepatitis C virus (HCV) infections were prospectively assessed in 356 heterosexuals with STDs (sexually transmitted diseases) and compared to a control group of 381 healthy first-time blood donors. Eighty-one of 356 STD patients were anti-HBC positive (22.8%) compared to 14/381 blood donors (3.8%; p less than 0.001). In addition, 18 of the 81 anti-HBC positive STD patients, but none of the controls, were positive for HBSAg (p = 0.06). The prevalence for anti-HCV was also significantly higher in the STD group than in the controls (5.3% vs. 0.5%; p less than 0.001). Among the various STDs syphilis (anti-HBC: 67.5%; anti-HCV: 12.5%) and Chlamydia trachomatis infections (anti-HBc: 20.2%, anti-HCV: 8.1%) had the highest prevalence for both infections. This study provides strong evidence of heterosexual transmission of hepatitis B and C virus infections. Thus, heterosexuals with STDs or multiple partners should be actively vaccinated against hepatitis B.