Background and Objectives: Drug-induced electrocardiographic QT interval prolongation is asociated with the occurrence of a potentialy lethal form of polymorphic ventricular tachycardia, termed torsades de pointes (TdP). Women are at greater risk for the development of drug-induced TdP. To determine whether this may be the result of gender-specific diferences in the efect of quinidine on cardiac repolarization, we compared the degree of quinidine-induced QT interval lengthening in young, healthy volunters. Subjects and Methods: Twelve women and 12 men each received a single intravenous dose of quinidine (4 mg/kg) or placebo in a single-blinded, randomized crosover trial. Total plasma concentrations of quinidine were measured, and QT and corected QT intervals were analyzed. Results: As expected, the mean QTc interval at baseline was longer for women than for men (443.6± 26.9 vs ± 31.3 msec, respectively, p= 0.037). The mean value of the maximal ΔQTc after quinidine infusion was higher in women (134.4± 46.4 vs 117.5± 37.7 msec, respectively, p= 0.029), and the mean value of the minimal ΔQTc for 1 hour after quinidine infusion was also higher in the female group (47.6± 15.7 vs 83.7± 25.4 msec, p= 0.034). However, there were no significant diferences in the time courses of the changes in the quinidine-in-duced QTc and ΔQTc interval between the two groups (p= 0.092, and p=0.305, respectively). Conclusion: Qui-nidine causes greater QT prolongation in women at equivalent serum concentrations. This diference may con-tribute to the greater incidence of drug-induced TdP observed in women taking quinidine, and has implications for other cardiac and noncardiac drugs that prolong the QTc interval.