The Moyamoya vascular convolutions are interpreted as an unspecific collateral system in unilateral and bilateral progressive stenosing of the A. carotis interna and additional transdural externainterna anastomoses. In such cases one should speak of a Moyamoya syndrome. The designation Moyamoya disease only applies to the congenital form. In an unclarified case observed by the authors, which however showed clinically and angiographically the typical findings of the Moyamoya syndrome, a haemodynamically effective anastomosis of the A. occipitalis externa to the A. vertebralis on the left side was also found for the first time in the series of transdural inflow variants.