PIP: The impact of enhanced syndromic diagnosis of symptomatic sexually transmitted infections (STIs) upon the incidence of HIV infections was evaluated in 8 paired villages in Mwanza, Tanzania, over a 2-year period. Shortly thereafter, a study was conducted in Uganda's Rakai district which focused upon treating all members of 5 clusters of paired communities, including those with symptomatic and asymptomatic STIs. In August 1995, the results of the Mwanza study showed that almost 40% of HIV infections had been prevented in the communities receiving the intervention. No other HIV intervention has had such a major effect upon infection rates. In contrast, however, no HIV infections were prevented in the Rakai intervention communities. The Mwanza results could reflect the short-term impact of STD prevention and control in an immature epidemic, while the Rakai study reflects the short-term impact in a mature epidemic. The probability of transmission, the duration of infectiousness, and the number of sex partners are discussed as factors which influence the generation of an HIV epidemic in a susceptible population. The 2 studies' results indicate that STD prevention and control is feasible, effective, and affordable.