The normal ultrastructure of a large cutaneous vein of the (hind) leg is described in an introductory chapter. This mentions, inter alia, the demonstration of acid mucopolysaccharides periodically attached to collagen fibrils, the immature character of most elastic fibres in the tunica intima and tunica media, and the mature aspect of elastic tissue in the tunica adventitia. Both varicose and non-varicose venous specimens of 60 patients with varicosis were examined. Two thirds of the varices were classified as belonging to the type which we have proposed to call C-type (C for collagen) with normally developed, mature elastic tissue in the tunica adventitia, but with foci of abnormal collagen fibrils in the vicinity of smooth muscle cells. The abnormal fibrils show swelling and helical splitting, resembling the findings of Peracchia and Vassallo (1966) in thromboangiitis obliterans. Collagen fibrils with periodical cloud-like attachments are also found. The relation of the latter to a possible coarsening of the periodically attached structures with acid groups remains unclear. Less than one third of the varices are described as the type which we have proposed to call E-type (E for elastic tissue) with an immature aspect of most elastic tissue of the tunica adventitia and only sparse abnormal collagen fibrils. The E-type cases are generally advanced varices, often with some sclerosis. Five mixed cases were found, showing characteristics both of the C-type and the E-type. Especially in patients with E-type varices, abnormal collagen fibrils were found in the non-varicose specimens, too. Such fibrils could not be demonstrated in the connective tissue of inguinal skin of varicosis patients. In non-varicose venae saphenae magnae of five coronary patients abnormal collagen fibrils were abundantly found. In a control case they occurred sporadically and in the vena tibialis of cattle not at all. In varicose and non-varicose specimens of the great saphenous vein of varicosis patients the vasa vasorum may show multiple laminae densae in the endothelial basement membrane. The discussion includes remarks on a possible pathophysiological enzymic influence of the smooth muscle cells on the surrounding connective tissue.