Vascular lesions of the right colon are being diagnosed increasingly as a cause of lower intestinal bleeding, but their nature and occurrence, primarily in the elderly, remains unexplained. Colons from patients with clinical and angiographic diagnoses of cecal vascular lesions were studied by injection and clearing, and by histological sections. In all injected specimens one or more mucosal vascular ectasias were identified. The mucosal lesions appeared to be secondary to dilated tortuous submucosal veins which were the more prominent feature and were often present without the mucosal ectasia. Possibly ectasias are caused by chronic, intermittent, low grade obstruction to submucosal veins with dilation and tortuosity initially of submucosal veins, then of venules, capillaries and arteries of the mucosal vascular unit. Ultimately, precapillary sphincters lose their competency, producing small arteriovenous communications. The concept that ectasias are degenerative lesions was evaluated by studying 15 right colons resected for carcinoma with no history of bleeding. Mucosal ectasias were identified in 4 colons and submucosal ectasias in 8. These lesions are apparently vascular ectasias developing as a degenerative process of aging. They are present with or without bleeding in a significant portion of the population over 60 yr of age. They are multiple more often than single, and may represent the most common cause of major lower intestinal bleeding in the elderly.