Pneumocystis carinii is an important, but sporadic, opportunistic pulmonary pathogen in immunosuppressed HIV seronegative persons. Historically, patients at highest risk for P. carinii pneumonia are included infants with severe malnutrition, children with primary immunodeficiencies, patients with hematological malignancies, and recipients of solid organ or bone marrow transplants. Recently, solid tumor patients, in particular those receiving high-dose corticosteroids for brain neoplasms, and patients with inflammatory or collagen-vascular disorders, especially patients with Wegener granulomatosis receiving immunosuppressive therapy, have been identified as subgroups at increased risk for P. carinii pneumonia. Other factors associated with P. carinii pneumonia include the intensity of the immunosuppressive regimen and tapering doses of corticosteroids. Because P. carinii pneumonia is associated with significant morbidity and mortality, it is important to identify high-risk patient populations to administer effective chemoprophylactic agents, such as trimethoprim-sulfamethoxazole.