Understanding the role of other sexually transmitted diseases (STDs) in the transmission of human immunodeficiency virus (HIV), the role of STDs in progression of HIV disease, and the role of HIV infection in alterations of natural history, diagnosis, or response to therapy of STDs is critical to the development of optimal strategies for HIV control. One hundred sixty-three studies on the interrelationships between HIV infection and other STDs were examined. Of 75 studies on the role of STDs in HIV transmission, the 15 analyses of examination or laboratory evidence of STDs adjusted for sexual behavior showed that both ulcerative and nonulcerative STDs increase the risk of HIV transmission approximately 3- to 5-fold. Due to limited data, the role of STDs in progression of disease remains unclear. Preliminary data from 83 reports on the impact of HIV infection on STDs suggest that, at a community level, HIV infection may increase the prevalence of some STDs (e.g., genital ulcers). If coinfection with HIV prolongs or augments the infectiousness of individuals with STDs, and if the same STDs facilitate transmission of HIV, these infections may greatly amplify one another. This "epidemiological synergy" may be responsible for the explosive growth of the HIV pandemic in some populations. Effective STD control programs will be essential to HIV prevention in these communities.