The purpose of this study was to assess cardiorespiratory responses to submaximal exercise in patients with univentricular atrioventricular connection after the Fontan operation, and to evaluate whether changes occur during medium-term follow-up. Eighteen patients (age 12.1±5.5 years) underwent graded exercise test on a treadmill 2.3±1.4 year after the Fontan repair. Ventilatory gases were measured using breath-by-breath analysis. Results were compared to gender/age-matched controls. Twelve patients (age 14.2±5.4 years) were reevaluated 2.4±2.1 years after the first test. Aerobic exercise performance was subnormal in all patients during the first test. At the lowest level of exercise, the ventilatory threshold was already surpassed in 6/18 patients, while it was reduced in all other patients (p<0.001). All patients were in stable sinus rhythm throughout the test. Heart rate at all exercise levels was ±10% below normal (p<0.05). The respiratory frequency was increased at all exercise levels (p<0.001). The ventilatory equivalent for oxygen was increased (p<0.001), and the end-tidal tension of carbon dioxide was decreased (p<0.001). The ratio of physiological dead space/tidal volume was increased in all patients (p<0.001), while the normal decrease of this ratio during exercise was not observed. Upon reevaluation heart rate, respiratory rate, oxygen uptake, venti latory equivalent for oxygen, end-tidal carbon dioxide tension and physiological dead space did not change signifi cantly. Only a slight further decrease in ventilatory threshold was observed. Aerobic performance after the Fontan procedure ranges widely from just above resting metabolic rate to the lower limit of normal. Dyspnea during exercise is exacerbated by a decreased ventilatory threshold, increased physiological dead space, and decreased respir-atory efficiency. Cardiorespiratory response to exercise, nonetheless, remains relatively stable during medium-term follow-up.