In this article, Pneumocystis carinii pneumonia (PCP) in persons with AIDS is described with regard to its epidemiology, pathogenesis, presentation, treatment, and prophylaxis. The changing epidemiologic patterns of PCP from 1981 to 1993 are diagrammed. Atypical and classical presentations of pre-AIDS, including extrapulmonary Pneumocystis, are discussed. Diagnostic strategies are outlined, giving algorithms for the most efficient means of diagnosing PCP. Various intravenous and oral treatment options for pneumonia, the use of adjunctive corticosteroids, and comparison of various prophylactic regimens are also presented in this article.