Patients who have undergone a Fontan-type operation usually have an elevated systemic venous pressure. To determine the sequelae of this nonphysiologic condition, we evaluated 66 patients 1 to 14 years after a Fontan-type operation. Fifty-one patients were apparently in good clinical condition, and 15 patients had symptoms and were restricted in their daily life. Bicycle exercise capacity, tested in 41 patients, ranged from 50% to 110% (mean 85%) of the predicted value for length. In 16 patients, a decreased capacity (< 85%) was, among others, related to arrhythmias and the presence of protein-losing enteropathy. A 24-hour ambulatory electrocardiogram was available in 56 patients and found to be normal in 32 (57%) patients. Arrhythmias were present in 21 patients, six of whom had symptoms. Three patients had previous pacemaker implantation. One or more abnormalities in liver enzyme and function tests were present in 40 patients (61%) and in coagulation factors in 46 patients (69%). The most pronounced was a protein C deficiency, a known thrombotic risk factor, present in 41 patients. The occurrence of arrhythmias increased with time of follow-up (p < 0.004), the occurrence of protein C deficiency decreased with time (p < 0.0001), and the occurrence of abnormal liver enzyme and function tests was not related to time of follow-up. With regard to age at operation, arrhythmias did not occur in patients who underwent operation at a mean age of 4 +/- 1.9 years (standard deviation), in contrast to patients who underwent operation at a mean age of 7.6 +/- 4 years (standard deviation) (p < 0.001). The occurrence of the two other types of sequelae was not related to the age at operation. With regard to the type of operation, only patients with a valved right atrium-to-pulmonary artery connection had a higher prevalence of arrhythmias than patients with a nonvalved or direct right atrium-to-pulmonary artery connection (p < or = 0.001). The latter patients also had a higher prevalence of protein C deficiency (p < or = 0.001). No relationship was found among the other types of operation, the underlying structure, or the hemodynamic condition measured at rest and the presence of arrhythmias, abnormal liver enzyme and function tests, or protein C deficiency. This point survey shows that even patients with an apparently good clinical condition are at risk for arrhythmias, abnormal liver enzyme and function tests, and coagulation factor abnormalities. Serial statement of affairs is recommended to ensure that adequate preventive measures can be taken.