The etiology of exercise-induced upper limb hypertension after repair of coarctation of the aorta is unknown. We hypothesized that blood flow across the coarctation repair site is a major determinant of such exercise-induced hypertension. Because arm ergometry should produce a smaller increase in descending aortic blood flow than treadmill exercise, we compared the changes in upper limb pressure and the coarctation gradient produced by each type of exercise at equivalent levels of heart rate and peak oxygen consumption in 28 children with repaired coarctation of the aorta. The children were classified into three groups: Group I, resting gradient less than 15 mm Hg and treadmill gradient less than 20 mm Hg; Group II, resting gradient less than 15 mm Hg and treadmill gradient greater than 20 mm Hg; and Group III, resting gradient greater than or equal to 15 mm Hg. Twelve children with no heart disease served as control subjects. All children were exercised to exhaustion with 45 minutes' rest between the two exercise protocols. There were no differences in maximal heart rate and oxygen consumption between the two types of exercise. In all groups, treadmill exercise produced a larger increase in arm systolic blood pressure and arm-leg gradient than did arm exercise. With treadmill exercise coarctation Groups II and III developed a greater rise in both arm-leg gradient and arm systolic pressure than was observed in the control subjects (p less than 0.05). However, with arm exercise, Group III developed a significantly greater rise in both arm pressure and arm-leg gradient (p less than 0.05) than was observed in the control subjects.