Fatty streak was always present in both the thoracic aorta and the abdominal aorta in the youngest subjects studied (aged 10-14 years). Fibrous plaque was present in a small proportion of these young subjects, but a rapid increase in prevalence occurred as early as the fourth decade. Complicated and calcified lesions appeared as early as the age of 20-25 years but a rapid increase in prevalence was seen after age 40 for complicated lesions and after age 50 for calcified lesions. There were differences in the prevalence of severe lesions among the five towns. There was little increase in the extent of atherosclerosis in the thoracic aorta before the age of 40 and in the abdominal aorta before the age of 20. The increase was more rapid after those ages. When atherosclerosis had affected about 50% of the intimal surface of the thoracic aorta and 70% of the intimal surface of the abdominal aorta, the increase slowed down considerably. In contrast to other types of lesion, the extent of fatty streak increased only up to 30 years of age, when it occupied 25-30% of the intimal surface. Then it declined and in the older age groups did not exceed 4-5% in men or women. The extent of fibrous plaque and complicated lesions was at all ages greater in men than in women, while the extent of fatty streak and calcified lesions in older age groups was greater in women. There were marked differences in the extent of atherosclerotic lesions in the five towns.