Pathology laboratories in nine cooperating centers collected arteries from 1532 persons 15 through 34 years of age who died of external causes, principally homicides, accidents, and suicides. A central laboratory stained the arteries and evaluated the atherosclerotic lesions. All of the aortas and about half of the right coronary arteries in the youngest age group (15 through 19 years) had lesions. The mean percent intimal surface involved by lesions, in 5-year age groups, increased from 15 through 34 years. Raised lesions increased with age in extent and prevalence in the aorta and the right coronary artery. Black subjects had more extensive fatty streaks than white subjects in all three arterial segments. Young women had more extensive fatty streaks in the abdominal aorta; young men had more in the thoracic aorta. Male subjects had more extensive and a higher prevalence of raised lesions than did female subjects in the right coronary artery. White and black subjects did not differ significantly in the extent of raised lesions. Among the three arterial segments, the right coronary had the least percentage of intimal surface involved with all types of lesions but had the highest proportion of raised lesions among total lesions. These results confirm the origin of atherosclerosis in childhood and show that the prevalence and extent of fatty streaks and fibrous plaques increase rapidly during the 15- through 34-year age span.