A 44-year-old man with acquired immunodeficiency syndrome (AIDS) and Pneumocystis carinii pneumonia (PCP) who suffered adverse effects from treatment with trimethoprim-sulfamethoxazole (TMP-SMX) and was then treated with pentamidine isethionate is described, and approved and investigational drugs used in the management of PCP in the AIDS patient are discussed. After taking TMP-SMX, 240 mg trimethoprim and 1200 mg sulfamethoxazole, four times a day orally for 10 days at home, the patient was hospitalized complaining of nausea, vomiting, diarrhea, and fever. Intravenous TMP-SMX was begun at a dosage of 18 mg/kg/day of trimethoprim. Four days later, his condition had deteriorated and he had elevations of liver enzymes and a decrease in white blood cell (WBC) count. TMP-SMX was discontinued and pentamidine isethionate was started at a dosage of 4 mg/kg/day i.v. His symptoms and fever subsided and his liver enzyme levels and WBC count improved. After nine days of pentamidine his WBC count decreased; pentamidine was suspected as the cause and discontinued; no further therapy was needed. PCP was the initial infection that established this patient's diagnosis of AIDS. The patient did not have exertional dyspnea and nonproductive cough, which are usually seen in AIDS patients with PCP. TMP-SMX 20 mg/kg/day, based on the trimethoprim content, is the usual initial treatment for PCP. Adverse effects of TMP-SMX develop more frequently in AIDS patients than in non-AIDS patients with PCP. The recommended dose of pentamidine isethionate for the treatment of PCP is 4 mg/kg/day, im. or i.v. A few studies have shown good response to aerosolized pentamidine. Trials of investigational agents have excluded patients with severely compromised respiratory status; eflornithine, dapsone in combination with trimethoprim, and trimetrexate have been used. Corticosteroids should be considered a last effort until additional data are available. TMP-SMX may be used to prevent recurrence of PCP or to prevent the initial occurrence of PCP in AIDS patients. Intravenous or aerosol doses of pentamidine may be effective as prophylaxis. Sulfadoxine-pyrimethamine tried as prophylaxis produced adverse reactions. Despite its higher incidence of serious adverse effects in the AIDS population, TMP-SMX is considered preferable to pentamidine for initial therapy. Pentamidine is preferred for patients with documented allergy to TMP-SMX or failure to respond to a five- to seven-day course of TMP-SMX.