Background: Previous studies suggest that concurrent sexually transmitted infection may enhance HIV transmission. This paper explores some theoretical consequences of this using a mathematical model of transmission of HIV and other STD pathogens. Objectives: To develop a deterministic mathematical model to describe the heterosexual transmission dynamics of both HIV and a bacterial STD. Study design: We used survey derived estimates of sexual behaviour in a young heterosexual London population in our deterministic mathematical model to estimate the eVects on an HIV epidemic of diVerent levels of STD prevalence in such a population. Results: We show that the predictions of the model are plausible and suggest that, even under conditions both of low STD prevalence and of low HIV transmission enhancement, a substantial proportion of HIV transmission events may be attributable to concurrent STD. Conclusions: It is likely that epidemics of heterosexually transmitted HIV infection in industrialised countries have been limited in size by the relative success of eVorts to control STD. None the less, a significant proportion of heterosexual transmission events which do occur may be attributable to concurrent STD. In developing countries, cheap and simple STD care is likely to be a highly cost eVective strategy to prevent HIV transmission. (Sex Transm Inf 1998;74:339-344)