OBJECTIVES: Life-threatening arrhythmias and prolonged QT interval have been reported in adults and children using cisapride, a medication structurally similar to procainamide. Premature infants have reduced activity of cytochrome p-450, the system responsible for metabolism of cisapride, which could lead to QT prolongation. Therefore, we prospectively studied premature infants and children receiving cisapride to analyze the effect of prolonged cisapride therapy on QT interval. DESIGN: Premature infants in a neonatal intensive care unit and children seen at a pediatric gastroenterology clinic in a tertiary care hospital had electrocardiography-analyzed and -corrected QT interval measured before cisapride (0.8/mg/kg per day) therapy, and again after 1 month of therapy. If baseline electrocardiography was not performed initially, it was obtained after cessation of therapy. RESULTS: A total of 30 children participated in the study. Mean corrected QT interval was similar at baseline and at 1 month after therapy. Significant QT prolongation was not found, and no adverse effects were recorded. CONCLUSIONS: Corrected QT interval during prolonged cisapride therapy at 0.8 mg/kg per day appears to be similar for premature infants and children. An inherent trend toward QT prolongation was not detected in either group. In the absence of other risk factors that alter cisapride metabolism or predispose to arrhythmia, cisapride may be safe for use in premature infants as well as in children. Additional studies are needed to confirm these data.