The first successful Fontan operation was performed in 1971, and this first cohort of Fontan patients is reaching adulthood with unclear outcome of this palliative procedure. We studied the mortality, morbidity, and quality of life in our adult Fontan patients. We examined all patients (n = 36) who underwent a Fontan procedure and were being seen in an adult outpatient clinic by using electrocardiography, exercise testing, and echocardiography. Quality of life was assessed by the Short Form 36 questionnaire. The mean follow-up period was 15 years (range 0 to 23). Of the initial 36 patients, 10 died (28%) at a mean of 10 years (range 0 to 21) after the Fontan operation and 1 patient underwent cardiac transplantation. Reoperations were performed in 21 patients (58%), and the most common reason was revision of the Fontan connection. Sustained supraventricular tachycardia was observed in 20 patients (56%) with an increased incidence of arrhythmias with longer follow-up. Thromboembolic events were detected in 9 patients (25%), 5 of whom had adequate anticoagulant levels at the time of event. The thromboembolic event was fatal for 3 patients. A total of 195 hospital admissions (mean 3.8 +/- 2.7, range 1 to 13) was recorded. Quality-of-life assessment showed physical functioning, mental health, and general health perception to be significantly lower for Fontan patients than for the normal Dutch population. Thus, we found high mortality and very high morbidity in adult patients after the Fontan operation. In particular, reoperations, arrhythmias, and thromboembolic events compromised quality of life.