The aim of this retrospective study was to determine the underlying diseases associated with Pneumocystis carinii pneumonia (PCP) in immunocompromised HIV-negative patients and to identify prognosis factors in this population. One hundred three cases of PCP were diagnosed over a 5-year period. Diagnosis was established on the basis of clinical features and by detection of Pneumocystis carinii cysts in bronchoalveolar lavage fluid. Underlying diseases comprised hematologic malignancies (n=60; 58%), inflammatory diseases (n=27; 26%), and solid tumors (n=18; 17.5%); 9 (8%) patients were solid organ transplant recipients. Seventy-one (69%) patients received cytotoxic drugs, 57 (55%) were treated with long-term corticotherapy, and 15 (14.7%) underwent bone marrow transplantation. Fifty-eight (56%) patients were admitted to the intensive care unit, and 52 (41%) required mechanical ventilation. Thirty-nine (38%) patients died of PCP; data from these patients were compared with those from surviving patients. The following factors were associated with a poor prognosis: high respiratory rate (P=0.005), high pulse rate (P=0.0003), elevated C-reactive protein (P=0.01), elevated serum lactate dehydrogenase level (P=0.02), and mechanical ventilation (OR, 14.4; 95%CI, 5-50). The results suggest that PCP can occur during the course of many immunosuppressive diseases, particularly various hematologic malignancies. The diagnosis of PCP should be considered more frequently and advocated earlier in immunocompromised HIV-negative patients, since prompt diagnosis may improve the prognosis of these patients.