The rapid increase in the worldwide prevalence of HIV infection and sexually transmitted diseases (STDs) has prompted research into the effect of STDs on HIV transmission as well as that of STD treatment interventions on HIV incidence. There is conclusive evidence that STDs act as co-factors for HIV transmission but STD treatment intervention studies have demonstrated contradictory results. Current evidence on the effect of STDs on HIV transmission and community-based randomised controlled trials of STD interventions in Uganda and Tanzania is discussed. Further prospective, randomised, community-based STD intervention studies focusing on the treatment of bacterial STDs and herpes simplex virus (HSV) infection are urgently needed in order to clarify the current uncertainty.