Background: Antiretroviral therapy has increased survival for individuals living with HIV and has led to an ageing of this population in developed countries. To date the rate of ageing has been unquantified, giving rise to uncertainty in the treatment emphasis and burden in this population. Methods: A mathematical model was used in conjunction with HIV/AIDS data from the Australian National HIV/AIDS Registry to estimate numbers and ages of Australian men who have sex with men (MSM) living with HIV infection from 1980 to 2005. Results: The average age of HIV-infected Australian MSM is estimated to exceed 44 years of age by the year 2010 and has increased by 1 year of age for each two calendar years since the mid-1980s. HIV-infected MSM over 60 years of age have been increasing in number by 12% per year since 1995. A consequence of successful therapy with subsequent ageing of those infected has meant that from 2001 estimated deaths from other causes exceed AIDS deaths in Australia. Conclusions: In summary, our analyses indicate an increasing and rapidly ageing population living with HIV in Australia. This will inevitably lead to more serious non-AIDS conditions in ageing patients living with HIV, and to increased treatment complexity.