Rates of newly acquired HIV infection are increasing in Australia, and sexual contact between men accounts for 85% of cases. To investigate behavioural risk factors for HIV seroconversion among gay and bisexual men from Sydney and Melbourne, Australia. 103 men with newly acquired HIV infection were recruited from clinics in Sydney and Melbourne and behavioural risk factor questionnaires were administered between January 2003 and October 2004. Results were compared with a cross sectional and a cohort study performed by our group that enrolled similar populations of men. The majority of seroconverters (73%) reported more than five sexual partners in the last six months. Ninety-five men (92%) were able to identify a high-risk event (HRE) that they thought had led to their HIV seroconversion. Most (70%) reported receptive unprotected anal intercourse (UAI), insertive UAI, or both at their HRE. Sixteen men (16%) reported no UAI in the preceding six months including the HRE. Men were more likely to report receptive UAI at their HRE when they perceived a partner to be HIV-negative as compared with when they perceived a partner to be HIV-positive (P = 0.05). Injection drug use was reported by 22% of the men in the previous six months, and 62% reported intoxication with alcohol (> or =5 drinks) or mood altering recreational drug use at the HRE. Gay and bisexual men who have recently seroconverted are highly sexually active and report high rates of unprotected anal intercourse and recreational drug use at the HRE.