A population of 622 prostitutes in Taiwan was tested for antibodies to the hepatitis C virus (anti-HCV) using a synthetic peptide assay composed of representative determinants from capsid and nonstructural (NS) viral proteins. Seventy-four (12%) were positive and the remaining 548 (88%) were negative. Seventy-nine samples were retested by a second-generation anti-HCV assay composed of recombinant capsid and NS proteins of HCV. Both assays had a nearly perfect agreement (Kappa value = 0.91). Of the positive cases, 31% were positive for reactivity to capsid only. Most (60/74, 81%) of the cases positive for synthetic peptide assay were HCV RNA positive, indicating potential infectivity. On the basis of the results of synthetic peptide assay, univariate analysis showed that history of paid sex for longer than 6 months, blood transfusion, acupuncture, intravenous drug abuse, and age over 20 years were significant risk factors of HCV infection (P < 0.01). Elevated alanine aminotransferase levels (> 40 U/L) were positively associated with anti-HCV, while the presence of serum hepatitis B surface antigen was a negatively associated factor. Multivariate analysis revealed that history of paid sex for longer than 6 months and blood transfusion were positively associated with anti-HCV (P < 0.001). The latter only accounted for less than one fifth of the HCV-infected prostitutes. This study indicates strongly that sexual transmission is an important route for HCV infection in prostitutes. This risk group may spread HCV to other populations as a sexually transmitted disease.