The chemistry, spectrum of activity, mechanism of action, pharmacokinetics, adverse effects, and dosage and administration of pentamidine are reviewed, and the role of the drug in treating Pneumocystis carinii infections in immunocompromised patients is discussed. Pentamidine isethionate, an aromatic diamidine compound, is active against certain protozoan organisms. Used extensively in the tropics in the treatment of Trypanosoma and Leishmania infections, its value in the management of Pneumocystis carinii infections has been demonstrated in infected immunosuppressed children and adults. Recently, interest in pentamidine has increased with the rising number of patients with acquired immunodeficiency syndrome (AIDS) who have P. carinii pneumonia. Pentamidine's mechanism of action and pharmacokinetic profile are not completely understood. Pentamidine is distributed extensively after i.v. or i.m. administration, with a volume of distribution of 3 L/kg. Appreciable quantities of pentamidine concentrate in the urine, and drug levels are detectable for up to six to eight weeks after cessation of therapy. After aerosol administration, the drug is almost exclusively recovered from the lung, with little extrapulmonary distribution. Available data suggest that approximately 50% of patients who receive the drug by the i.v. or i.m. route will experience some drug toxicity (local pain, sterile abscesses at the intramuscular injection site, hypoglycemia, hypotension, or azotemia), while adverse effects after aerosol therapy include bronchial irritation but little systemic toxicity. Regardless of the route of administration, pentamidine has emerged as a mainstay of therapy in the management of P. carinii pneumonitis in AIDS patients, especially in those who are allergic to the sulfa component of trimethoprim-sulfamethoxazole.