After successful Fontan surgery, the risk of mortality in childhood is only low. Unfortunately, some of the patients suffer from Fontan-typical long-term complications in the long-term course, whereby protein loss neuropathy must be mentioned in particular, which is described in the literature with an incidence of 3-14% (1, 2) and still has a 5-year risk of death of 6-12% today (2, 3). Protein loss tereopathy leads to loss of protein in the intestine and subsequently to diarrhea and edema. Other problems concern the liver, which can develop cirrhosis due to chronic congestion (4-6). Cardiac can lead to heart failure and arrhythmias. The registry study described in this protocol is intended to identify factors that influence the treatment outcome of patients in the Fontan circulation in the long term through systematic prospective documentation of the data from our standardized and guideline-oriented treatment.