An aneurysm in the region of the dilatation occurred in two of 23 patients (a 17-year-old boy and a 29-year-old woman) three and one years, respectively, after balloon dilatation for coarctation of the (thoracic) aorta. The aneurysm in the boy was small (18 mm diameter) and he was symptom-free so that no operation is as yet indicated. But the woman had progressively increasing backache after physical exertion and the aneurysm was 4 cm in diameter. Therefore, the aneurysm and coarcted segment were resected and a Dacron prosthesis interposed. As the ultimate value and risk of balloon dilatation of coarctation of the aorta cannot as yet be definitively judged, all patients should be carefully and repeatedly examined over a longer period.