The relation between the presence of calcified plaques in the thoracic aorta, as detected on chest x-rays, and the development of cardiovascular disease is examined during 12 years of follow-up of the Framingham cohort (n = 5,209). The prevalence of aortic calcified plaques approximately doubled with each decade of age, with only a trivial male predominance. Its presence was associated with a twofold increase in risk of cardiovascular death in men and women younger than age 65, even after other risk factors were taken into account. Similar increases in risk were found for coronary artery disease, stroke and intermittent claudication among middle-aged women. In middle-aged men these risks were less marked. The predictive value of aortic calcified plaques generally diminished with age. Risk of sudden coronary death in men with calcified plaques in the thoracic aorta ranged from a sevenfold increase at age 35 to no excess risk at age 70 years. These results support the view that atherosclerosis is a generalized process. The finding of aortic calcified plaques in a relatively young subject on a routine chest x-ray should be regarded as a sign for potential development of clinically manifest atherosclerotic disease in the cardiac, cerebral and peripheral arterial circulation.