OBJECTIVES: To measure the determinants of syphilis among female sex workers (FSWs) in the state of Karnataka, South India. METHODS: During 2004-2006, cross-sectional surveys were administered to 2312 FSWs across 5 districts in the state, in the context of a large-scale HIV preventive intervention program. Demographic and behavioral information, and serum (for syphilis, HSV-2 and HIV) and urine specimens (for Neisseria gonorrhoeae and Chlamydia trachomatis) were obtained. RESULTS: The prevalences of lifetime (TPHA positive) and active (RPR and TPHA positive) syphilis were 25.3% and 9.6%, respectively. There was considerable variation in the prevalence between districts, ranging from 10.9% to 37.4% lifetime, and 3.4% to 24.9% active infection. Factors associated with lifetime syphilis were older age, longer duration of sex work, illiteracy, client volume, practising sex work in >1 city, and sex work typology (public solicitation followed by brothel or lodge-based sex). The same typology, client volume, illiteracy, and having been widowed, divorced or deserted, were predictive of active infection. Of the 976 women who had symptoms of an STI, 78.8% had sought medical treatment, behavior that was protective for both outcomes. HIV infection was strongly associated with lifetime (OR 2.0; 95% CI: 1.6-2.6) and active syphilis (OR 2.1; 95% CI: 1.5-2.9). CONCLUSIONS: Despite reasonable treatment-seeking behavior, the high prevalence of syphilis has necessitated enhanced outreach efforts for FSWs and acceleration of the implementation of syphilis screening. Mobilizing resources to enhance syphilis control will not only reduce the burden of syphilis morbidity, but should impact in reducing HIV transmission.