Necropsy findings in patients with Down's syndrome have suggested an absence of atherosclerosis throughout the cardiovascular system, but there are also contradictory results. We compared the left coronary arteries of 15 institutionalised Down's syndrome patients (5 males, 10 females, mean age 51 years) with those of 6 other institutionalised mentally retarded patients (4 males, 2 females, mean age 49) and 20 normal, free-living subjects (10 males, 10 females, mean age 48) by macroscopic inspection of the opened coronary arteries and by biochemical analysis of their intima-media. The arteries of Down patients contained a lower percentage of raised lesions and less calcium than the arteries of the control groups. Thus, even though the coronary arteries of mongoloids were not completely free of atherosclerosis, it was milder than in other mental patients and free-living subjects of the same age.