Three-hundred and ten cases of vascular malformations have been analyzed on the basis of pathomorphological examination by light and electron microscopy and by immunohistology. The findings demonstrate that all the vascular malformations have a common pathogenesis. Differences are only gradual and always follow the categories: agenesis-aplasia-hypoplasia-dysplasia-hyperplasia. Additional degenerative alterations are secondary. Hemangiomas in connection with angiodysplasias are congenital malformations and not tumors. On the level of differentiated vessels they mostly contain arteriovenous macro- or microshunts (a-v hemangiomas); if only capillaries are involved (cavernous hemangiomas), a-v microshunts are usually absent. Classification of angiodysplasias should be simplified and based on a pathomorphological concept. The descriptive diagnosis should include size and type of vessels involved, kind of alteration, presence of hemangiomas with or without a-v shunts and systemic involvement.