We evaluated the effect of chronic Pneumocystis carinii pneumonia (PCP) prophylaxis, with a once a month dose of 300 mg of inhalatory pentamidine isethionate, on QT interval duration. We included 22 human immunodeficiency virus (HIV)-infected patients: 11 were on this medication and 11 were not. The two groups were matched for age, sex and HIV infection stage. No patient had any clinical condition or was under any medication known to affect the duration of the QT interval. The heart rate-corrected QT (QTc) was obtained by averaging the observations of three independent observers. QTc duration was similar in both groups. The time separating pentamidine administration and the performance of the ECG did not influence the results, neither did the duration of inhalatory pentamidine therapy. Our results suggest that inhalatory pentamidine does not prolong the QT interval duration and so, as opposed to what has been reported concerning intravenous pentamidine therapy, does not seem to induce an increased risk of torsades de pointes.