Abstract An age-specific transmission model is proposed to describe the spread of HIV in a homesexual population. The model incorporates developments in the treatment of patients and the change in the surveillance definition of AIDS. The model is applied to the Australian epidemic with the aim of determining the extent of behavioral changes during the epidemic and assessing the extent to which therapy has changed the course of the epidemic. It is found that therapy and the adoption of totally safe sex practices by individuals who have tested HIV positive cannot explain the recent downturn in the rate of increase of observed AIDS incidence. A significant change in behavior within the general homosexual community is indicated.