Muscle biopsies from 8 children with dermatomyositis and from 4 normal children and 4 children and adults with other primary diseases of muscle were studied by ultrastructural techniques. The major abnormalities in dermatomyositis of childhood were in the walls of the intramuscular blood vessels, more specifically in the endothelial cells of capillaries, arterioles, and veins. Endothelial cells were observed in various stages of degeneration and regeneration. In a single plane of section, inclusions consisting of tubular aggregates were found within the cytoplasm of 76 to 98% of all intramuscular blood vessels. The angiopathy resulted in multiple thrombus formation and infarction of muscle. Three factors were of particular importance in the genesis of these events: separations at the cell junctions in the regenerating endothelium; the necrosis of endothelium with subsequent retraction, distortion and cell loss; and endothelial hyperplasia. These observations provide strong support for the concept that dermatomyositis of childhood is a unique disease, the fundamental lesion of which is in the walls of the intramuscular blood vessels.