In the 1980s, tens of thousands of cases of Pneumocystis carinii pneumonia (PCP) occurred in persons with AIDS, making it the most common opportunistic infection encountered in this patient population. Thus, the presentation and clinical course of PCP became well-known to many physicians. Overshadowed by this epidemic was the continued occurrence of PCP in persons not infected with the human immunodeficiency virus, most notably those receiving immunosuppressive therapy for neoplastic disease or other indications. A review from Memorial Sloan-Kettering Cancer Center has demonstrated that, in addition to those patients previously defined as at risk--those with acute lymphocytic leukemia or allogeneic bone marrow transplantation--patients receiving corticosteroid therapy for a primary or metastatic brain neoplasm are at risk for PCP and should receive prophylaxis. Among patients with neoplastic disease, the disease may be more fulminant and the mortality rate higher--approaching 50% in most series. Wider use of prophylaxis should decrease the frequency of this disease, while prompt initiation of therapy in patients with a compatible syndrome should help to lower mortality rates.