Whereas new, or changes in existing, routes of transmission of HIV have not been identified in the 11 years since AIDS was identified as a clinical syndrome, changes in the epidemiology of HIV infection in the US have been identified during that period. The role of injection drug use as a risk for both parenteral and sexual transmission of HIV has increased substantially during this period. Heterosexual transmission is becoming more prominent as the epidemic continues to "mature" in the US. The likelihood that heterosexual transmission will become progressively more important in the spread of HIV in the US in the next several years seems high. The ability of individuals in some populations at risk for infection to modify risk behaviors has led to a reduction in transmission of HIV in those populations. The addition of nucleoside analog antiretrovirals and effective chemoprophylaxis for Pneumocystis carinii pneumonia has led to increases in both the quality and duration of life for some populations of HIV-infected patients. Neither a chemotherapeutic cure nor a vaccine is on the immediate horizon; education and behavior modification remain the cornerstones of current prevention efforts. For a variety of complex reasons, inappropriate scrutiny has been focused on the remote risks of health-care provider-to-patient transmission of HIV. In the past 11 years medical science has made remarkable progress in understanding the etiology, biology, epidemiology, pathogenesis, and prevention of HIV infection. Despite this progress, a great deal of work remains to be done not only in the medical and basic science arenas but also in the behavioural and sociological sciences.