Eight patients, 9.1 to 16.5 years of age, were studied 2.8 to 8.5 years after Fontan operation. Oxygen utilization was determined during upright bicycle exercise. The cardiac index and stroke index were measured by echocardiography and the anaerobic threshold was determined. The results were compared with 10 patients after surgical closure of the atrial septal defect. Anaerobic threshold (AT) in Fontan patients was lower than in the control subjects. Oxygen consumption at each stage of exercise was significantly lower in the Fontan group compared with the control subjects. From the beginning of exercise until AT, the increase in stroke index was lower in the Fontan patients than in the control subjects. After that point, the stroke index decreased significantly in the Fontan patients while it remained almost at the same level in the control subjects. Significant correlations were observed between the oxygen pulses and the stroke index at AT both in the control and Fontan groups. These results suggest that impaired exercise capacity in Fontan patients is mainly due to a subnormal response of the stroke index at AT and to the decreased response of the stroke index and the heart rate at the maximal workload.