AbstractAs part of the worldwide effort involved in modelling the spread of AIDS, nothing has been heard from the operational research community. Yet OR, with its client-oriented modelling philosophy, patently has a role to play in assisting clinicians, epidemiologists and health planners to obtain greater understanding of this staggering threat to public health. Using the methods of system dynamics, a model of the spread of AIDS in the UK homosexual population has been developed. It is implemented on a personal computer, and possesses the capability to capture complex virological and behavioural features of the epidemic whilst portraying the consequences in an easily digested graphical form. Some examples of these results are presented. The model's structure and parameters are also fully described, together with sensitivity tests on the crucial incubation time distribution.