Systolic blood pressure was measured at rest and during exercise in 43 children who had undergone operation for correction of coarctation of the aorta, five children awaiting surgery for coaractation, and 22 control children. Ages ranged from 2 to 15 years, mean 7-6 years. The mean blood pressure of children with coarctation in both the pre- and postoperative groups was significantly higher at rest than in the controls. Of 43 postoperative patients, 15 (35%) were hypertensive (systolic blood pressure more than 95th centile), and 12 of these had a gradient between the upper and lower limb. Seven of the 28 normotensive patients also had a gradient postoperatively. Exercise increased the blood pressure more in children with coarctation than in controls, but there was much individual variability and this difference was not significant. Some children with coarctation developed very high blood pressures on exercise, but this was not related to the presence of a gradient. The mean interval after operation was significantly shorter in the hypertensive group, independent of the age at operation. Hypertension with or without a gradient commonly persists despite apparent successful surgical correction, but exercise is of limited value in its assessment in this age group.