BACKGROUND: The majority of sex work in India is clandestine due to unfavorable legal environment and discrimination against female sex workers (FSWs). We report data on who these women are and when they get involved with sex work that could assist in increasing the reach of HIV prevention activities for them. METHODS: Detailed documentation of demography and various aspects of sex work was done through confidential interviews of 6648 FSWs in 13 districts in the Indian state of Andhra Pradesh. The demography of FSWs was compared with that of women in the general population. RESULTS: A total of 5010 (75.4%), 1499 (22.5%), and 139 (2.1%) street-, home-, and brothel-based FSWs, respectively, participated. Comparison with women of Andhra Pradesh revealed that the proportion of those aged 20-34 years (75.6%), belonging to scheduled caste (35.3%) and scheduled tribe (10.5%), illiterate (74.7%), and of those separated/divorced (30.7%) was higher among FSWs (p < 0.001). The FSWs engaged in sex work for >5 years were more likely to be non-street-based FSWs, illiterate, living in small urban towns, and to have started sex work between 12-15 years of age. The mean age at starting sex work (21.7 years) and gap between the first vaginal intercourse and the first sexual intercourse in exchange for money (6.6 years) was lower for FSWs in the rural areas as compared with those in large urban areas (23.9 years and 8.8 years, respectively). CONCLUSION: These data highlight that women struggling with illiteracy, lower social status, and less economic opportunities are especially vulnerable to being infected by HIV, as sex work may be one of the few options available to them to earn money. Recommendations for actions are made for long-term impact on reducing the numbers of women being infected by HIV in addition to the current HIV prevention efforts in India.