We investigated the prevalence of hepatitis B antigen (HBAg) and antibody (HBAb) in 293 prostitutes and in 379 pregnant women of similar age and of low socioeconomic level, who served as controls. HBAg was found in 4.4% of prostitutes and 3.4% of controls. The prevalence of HBAb was significantly higher (P <0.001) in prostitutes (56.7%) than in controls (24.5%). The prevalence of HBAb was clearly age-dependent in both groups. Evidence of hepatitis B virus infection significantly increased with the number of years in prostitution. The evidence of increased infection rates among prostitutes and their distribution support the hypothesis that hepatitis B infection is sexually transmitted.