Pneumocystis carinii pneumonia (PCP), the most common presenting manifestation of the acquired immunodeficiency syndrome (AIDS), is a major and recurring cause of morbidity and mortality for persons infected with the human immunodeficiency virus (HIV). In recent years, important advances have been made in understanding which patient subpopulations are at highest risk for developing PCP and in the design of chemotherapeutic regimens that can reduce the frequency of this illness. Recently, a number of experts convened by the National Institutes of Health independently reviewed data on prophylaxis against PCP among persons infected with HIV, and then provided recommendations to the U.S. Public Health Service concerning which persons should receive prophylaxis and what specific prophylactic regimens should be used. The resulting guidelines are detailed below.