Note From Dr. Merle A. Sande—We have recently come to appreciate the fact that successful chemotherapy of opportunistic infections in patients with AIDS is akin to treatment of certain malignancies: In both instances, induction, followed by maintenance, therapy is required. Although control of initial infections is usually successful with current regimens, relapse is common if lifelong prophylactic or suppressive therapy is not used. This is true in human immunodeficiency virus (HIV)-infected individuals with Pneumocystis carinii pneumonia, cryptococcosis, histoplasmosis, toxoplasmosis, and cytomegaloviral infection. Because Drs. Joseph A. Kovacs and Henry Masur of the National Institutes of Health have been instrumental in developing strategies for the management of P. carinii pneumonia, they were asked to discuss their current approach to chemoprophylaxis of this infection in this AIDS Commentary.