Pneumocystis carinii pneumonia (PCP) emerged in the 1980s as the most common opportunistic infection among patients with the acquired immunodeficiency syndrome (AIDS). Because of this, the presentation and clinical course of PCP has become well-known to many physicians. However, PCP continues to occur among patients not infected with the human immunodeficiency virus, generally those who receive immunosuppressive therapy as treatment for neoplastic disease. A review from Memorial Sloan-Kettering Cancer has shown that a new group of patients, those receiving corticosteroid therapy for brain neoplasm, are also at risk for the development of PCP and should receive PCP prophylaxis. Previously defined patient groups--people with acute lymphocytic leukemia or allogeneic bone marrow transplantation--also should continue to receive prophylaxis. In addition, the clinical course and outcome of patients with neoplastic disease who develop PCP may differ from those with AIDS and PCP: the disease may be much more fulminant among patients with neoplastic disease, and the mortality rate much higher, approaching 50% in the Memorial Sloan-Kettering Cancer Center series. Wider use of prophylaxis should decrease the frequency of this disease, whereas prompt initiation of therapy in patients with a compatible syndrome should help to improve mortality rates.