Although pentamidine isethionate is effective in the treatment of Pneumocystis carinii pneumonia, it frequently causes serious adverse reactions. We report a case of reversible pentamidine-induced cardiotoxicity, characterized electrocardiographically by prolongation of the QT interval, T-wave inversion, and electrical alternans of the U-wave. In addition, the patient had repeated episodes of ventricular tachycardia that culminated in torsades de pointes. Our case re-emphasizes the need for close patient monitoring during pentamidine therapy.