A 27-year-old man, HIV-positive for 4 years, developed ventricular fibrillation and cardiac arrest during treatment of Pneumocystis carinii pneumonia with intravenous pentamidine isethionate. The dosage was 4 mg/kg/day for 18 days. Nephrotoxicity occurred and raised serum potassium. The plasma concentration of pentamidine was 580 nmol/l. Careful monitoring of renal and cardiac functions is recommended during intravenous therapy with pentamidine isethionate.