BACKGROUND: The identification of factors associated with delayed diagnosis of HIV infection in Victoria, Australia was the aim of the present study. METHODS: Demographic and epidemiological characteristics of cases notified to the Victorian HIV surveillance database between 1 January 1994 and 31 December 2006 were analysed. Delayed diagnosis was defined as: CD4 count below 200 cells mm(-3) at HIV diagnosis or diagnosis of AIDS earlier than 3 months after HIV diagnosis. RESULTS: Diagnosis of HIV was delayed in 627 (22.6%) of 2779 cases. Of these, 528 (84.2%) had either a high-risk exposure or were born in a high-prevalence country. The most common exposure was male homosexual contact in 64.3% of cases. Independent risk factors for delayed diagnosis were: older age at diagnosis (30-39 years odds ratio [OR] 2.15, > or = 50 years OR 7.50, P < 0.001), exposure via routes other than male homosexual sex or injecting drug use (heterosexual sex OR 2.51, P < 0.001, unknown/other route OR 4.24, P < 0.001); birth in Southern/Eastern Europe (OR 2.54), South-east Asia (OR 2.70) or the Horn of Africa/North Africa (OR 3.71, P < 0.001), and male gender (OR 0.47 for females, P < 0.001). CONCLUSION: Delay in the diagnosis of HIV infection is common in Victoria, but potentially avoidable in the majority of cases. Most people with delayed diagnosis had a history of male homosexual contact, injecting drug use, birth in a high-prevalence country or sexual contact with such individuals. An accurate sexual history, together with knowledge of their country of birth, should identify most individuals who should be offered an HIV test.