We compared radial and femoral arterial blood pressures in 29 patients, ranging in age from 1.25 to 17 years, during and after cardiopulmonary bypass for repair of congenital heart disease. Radial mean arterial pressure (MAP) was more than 10% lower than femoral MAP in 17 patients (58%), and in 7 of these patients (24%) radial MAP was more than 20% lower than femoral MAP. In 27 of 29 patients (93%) systolic radial pressure was 10% lower than systolic femoral pressure, and in 20 of these (69%) it was more than 20% lower. The ratio of radial to femoral pressure correlated with MAP (i.e., lower MAP produced greater differences), and the ratio of systolic radial to systolic femoral pressure inversely correlated with systemic vascular resistance index. We found no correlation between femoral-minus-radial pressure difference and postoperative course. These data demonstrate that radial arterial pressure may be misleadingly low in children undergoing operation for correction of congenital cardiac defects.