Pneumocystis carinii pneumonia in patients with primary brain tumors. All histologically documented episodes of Pneumocystis carinii pneumonia in adult patients with primary brain tumors treated at Memorial Sloan-Kettering Cancer Center, New York, NY, since 1981, were retrospectively reviewed. Pneumocystis carinii pneumonia was histologically documented 11 times in 10 patients. During the same 8-year interval, approximately 587 adults were seen at the center for a brain tumor, 90% of whom received ongoing therapy. Therefore, in at least 1.7% (10/587) of our patients with brain tumors, P carinii pneumonia developed. The median duration of dexamethasone therapy at the onset of P carinii pneumonia symptoms was 2.75 months. Symptoms began during tapering of steroid therapy in eight episodes. Bronchoscopy was diagnostic in the eight cases in which it was performed. Four episodes (40%) were fatal. Trimethoprim-sulfamethoxazole prophylaxis may be indicated in some patients with brain tumors, especially during tapering of steroid therapy. 