One high risk group for hepatitis C virus (HCV) infection is female sex workers (FSWs). A study of HCV antibody prevalence and group risk factors was conducted in 200 FSWs in Ratchaburi Province, Thailand, during June to December 1995. FSWs were interviewed and their blood specimens were collected for determining HCV antibody by second generation EIA (ABBOTT). After the laboratory results, the FSWs were divided into 2 groups, anti-HCV positive and anti-HCV negative. The variables obtained from interviews were analysed by X2-test and Odds Ratio. Prevalence of HCV antibody positives was 9.5%. It tended to be increased by the longer duration of working in the sex trade. The anti-HCV prevalence of FSWs working 9 years or more in the sex trade was significantly higher than that of FSWs working 4 years or less by about 3.5 times (23.08% vs 6.67%, p = 0.008). The anti-HCV positive FSWs had a higher percentage of anti-HIV positivity, but it was not of statistical significance (p = 0.078). The factors associated with high risk for HCV infection in this group were (a) Domicile (Northeast): OR = 3.07, (p = 0.0182), (b) Duration of working (> or = 4 years): OR = 3.13 (p = 0.0216), (c) Having a tattoo: OR = 4.12 (p = 0.0406), and (d) Having a history of STDs in the last 4 years: OR = 3.46 (p = 0.0165).