Pneumocystis carinii pneumonia is a devastating disease state that is almost always fatal without appropriate therapy. The offending organism is difficult to culture, and the diagnosis is best confirmed by performing an open-lung biopsy. Drug therapy includes pentamidine in doses of 4 mg/kg/d im and/or trimethoprim/sulfamethoxazole (TMP/SMX) in doses of 20 mg/Kg/d of the former and 100 mg/kg/d of the latter in four divided doses. Of these two agents, TMP/SMX appears to be most effective, although pentamidine may be of benefit if the combination product fails. Adverse effects associated with pentamidine are greater in number and more severe than those associated with TMP/SMX. A case report is presented to demonstrate drug therapy and the severity of the disease.