BACKGROUND: HIV notifications affecting men having sex with men (MSM) in Victoria, Australia have been increasing. This study aimed to determine current risk factors for HIV infection in this population. METHODS: Case-control study. Cases were MSM infected within the previous year (incident cases) as indicated by a previous negative test or seroconversion illness. Controls were MSM with a negative HIV test at the same clinic. From May 2001 to May 2003, cases and controls were interviewed about sexual behaviour, drug and alcohol use and mental health and sexually transmissible infections (STI) in the year before their HIV diagnosis. RESULTS: Twenty-six cases and 52 controls were recruited. Risk factors in the year before diagnosis of incident HIV infection included: receptive unprotected anal intercourse (UAI) with ejaculation with casual partners (odds ratio [OR] and 95% confidence interval 57.2 [6.7, 489.4]); insertive UAI with ejaculation with >1 casual partners (OR 19.2 [2.2, 168.9]); having >14 casual partners at sex venues (OR 3.2 [1.1, 9.1]); and consuming >60 g alcohol at one sitting at least weekly (OR 3.6 [1.1, 11.4]). Cases were also more likely to have anal sex with >100 partners in their life and cases had more casual partners than controls in the year before the test. Cases were more likely to have consumed alcohol or amphetamines during a high-risk sexual episode in the year before the test. CONCLUSIONS: UAI remains the most important behavioural risk for HIV in Australian MSM. Risk is increased by larger numbers of partners, partners met at sex venues and sex under the influence of alcohol.