BACKGROUND: HIV prevention in India has mostly focussed on heterosexual transmission. Data on homosexual transmission are not readily available from India. We therefore assessed the probability of acquiring and transmitting HIV for men who sell sex to men and compared this with women who sell sex in India. METHODS: Sexual behaviour characteristics of 6661 men who have sex with men and 6648 women who sell sex were obtained in the Indian state of Andhra Pradesh through confidential interviews. These, along with estimates of HIV rates among them and risk of HIV transmission per unprotected sex act from other sources, were used to calculate their annual probability of acquiring and transmitting HIV. RESULTS: Of 6661 men who have sex with men in this sample, 1776 (26.7%) had sold sex to men. For every 1000 men who sell sex to men, annually 146 (95% confidence interval [CI] 116-179) would acquire HIV and HIV would be transmitted to 55 (95% CI 42-71) men who do not sell sex or women. These estimates were higher by 6.7 (95% CI 4.9-9.2) times for acquiring HIV and 2.5 (95% CI 2.0-3.2) times for transmitting HIV to sex partners outside their group, as compared with similar estimates for women who sell sex. In this sample, the average annual probability of acquiring HIV was higher among men who have sex with men but do not sell sex as compared with women who sell sex. CONCLUSION: These data indicate that men who sell sex to men are at much higher risk of acquiring and transmitting HIV than women who sell sex. Therefore, men who sell sex to men and their clients warrant substantial attention for comprehensive HIV prevention in India.