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Objective-To estimate per-contact probability of HIV transmission in homosexual men due to unprotected anal intercourse (UAI) in the era of highly active antiretroviral therapy (HAART).Design-Data were collected from a longitudinal cohort study of community-based HIV-negative homosexual men in Sydney, Australia.Methods-A total 1427 participants were recruited from June 2001 to December 2004. They were followed up with 6-monthly detailed behavioral interviews and annual testing for HIV till June 2007. Data were used in a bootstrapping method, coupled with a statistical analysis that optimized a likelihood function for estimating the per-exposure risks of HIV transmission due to various forms of UAI.Results-During the study, 53 HIV seroconversion cases were identified. The estimated per-contact probability of HIV transmission for receptive UAI was 1.43% (95% CI 0.48%-2.85%) if ejaculation occurred inside the rectum occurred, and it was 0.65% (95% CI 0.15%-1.53%) if withdrawal prior to ejaculation was involved. The estimated transmission rate for insertive UAI in participants who were circumcised was 0.11% (95% CI 0.02%-0.24%), and it was 0.62% (95% CI 0.07%-1.68%) in uncircumcised men. Thus, receptive UAI with ejaculation was found to be approximately twice as risky as receptive UAI with withdrawal or insertive UAI for uncircumcised men and over 10-times as risky as insertive UAI for circumcised men.Conclusion-Despite the fact that a high proportion of HIV-infected men are on antiretroviral treatment and have undetectable viral load, the per-contact probability of HIV transmission due to UAI is similar to estimates reported from developed country settings in the pre-HAART era. KeywordsHIV; per-contact probability; transmission risk; cohort study; homosexuality, male; Australia