3 patients with a congenital pulmonary arteriovenous fistula are presented. The clinical signs of this condition are dependent on the size of the functional right to left shunt. The symptoms worsen with increasing arterial hypoxemia. Due to the hemodynamics of arteriovenous fistulas signs of cardiac hypertrophy are usually not seen on ECG of X-ray examination. However, in almost all cases pulmonary X-ray shadows are found. In addition, nearly 50% also have an hereditary hemorrhagic telangiektasis. The diagnosis must be verified in every case by cardiac catheterisation with angiography, since only exact anatomic data can give precise information on whether or not an operation is possible.