Using the electron microscope, the authors examined the structure of the wall in a case of arteriomegaly of the inferior mesenteric artery in man. Structural changes concerned the elastic material showing signs of degeneration. Based on a comparison with literary findings, the authors conclude that tortuosity of the arteries, aneurysms and arteriomegaly are caused by congenital, age-dependent and pathological changes in the elastic material in the vessel wall. There exist basic structural differences in the arterial wall in arteriomegaly and in atherosclerosis.