Background: The aim of Fontan procedure is to restore a balance between pulmonary and systemic circulation and improve or nearly normalize arterial saturation in patients with functionally univentricular heart. Nevertheless, due to the lack of subpulmonary pump, the circulatory system becomes haemodynamically less efficient, what can lead to the impairment of exercise capacity. Aim: The aim of the study was to investigate exercise capacity by means of cardiopulmonary exercise test and pulmonary function tests, of 37 adult Fon tan patients with regard to the time passed from the index procedure. The patients were divided into 3 groups: group I – up to 15 years post procedure; group II – 16–20 years, group III – more than 20 years after Fontan procedure). Results: 37 Fontan patients (mean age was 24.4 ±5.7 years, 40% of women) were enrolled in the study. The mean postoperative time was 19.4 ±5.1 (13–30) years. Mean, peak oxygen consumption (VO2peak) was 22.7 ±7.1 ml/kg/min (64.2 ±18.5% of predicted value). According to postoperative time a significant increase of ventilatory equivalent of carbon dioxide (VE/VCO2 ) (p = 0.033) and significant decrease of forced expiratory volume in 1 s/ vital capacity (FEV1 %VC) between group 2 and 3 (p = 0.026) were observed. Additionally, the age of the patients correlated negatively with and heart rate (HR) (r = -0.360, p <0.05) and peak oxygen consumption (VO2peak) (r = -0,337, p < 0.05). Moreover, age at Fontan opera tion and time after Fontan procedure was related to ventilator equivalent of oxygen (VE/VO2 ) (r = -0,343, p <0.05, and r = 0.393, p <0.05). Single ventricle ejection fraction (SVEF) and atrioventricular regurgitation degree did not corresponded with cardiopulmonary exercise test (CPET) values. Conclusions: Results highlight the complex problem of diminished exercise capacity of Fontan patients depending on the time passed from the procedure. Exercise tolerance deteriorates in time: VE/VCO2 increases, FEV1/VC markedly lowers in a group of Fontan patients at 20 years follow-up