The Pathobiological Determinants of Atherosclerosis in Youth (PDAY) study was organized to document the natural history of athersclerosis and to determine the relation of cardiovascular risk factors to atherosclerosis in young subjects. Pathology laboratories in 15 centers collected coronary arteries, aortas, and other tissues from over 3,000 subjects age 15 to 34 who died of external causes between 1987 and 1994. The extent, prevalence, and topography of arterial lesions were evaluated and risk factors were analyzed in a central laboratory. Postmortem risk factors included serum lipoproteins, serum thiocyanate (smoking), glycohemoglobin (diabetes), thickness of panniculus adiposus and body mass index (obesity), changes in small renal arteries (hypertension), and apoprotein isoforms. The PDAY study confirmed the origin of atherosclerosis in childhood, showed that progression toward clinically significant lesions may occur in young adulthood and demonstrated that the progression of atherosclerosis is strongly influenced by coronary heart disease (CHD) risk factors. Recent PDAY studies have shown that a significant number of advanced coronary artery lesions have microscopic qualities associated with susceptibility to rupture and that CHD risk factors are associated with the development of these characteristic microscopic qualities. The PDAY archive continues to provide an important resource for new investigators throughout the world that contribute to the understanding of atherosclerosis, the underlying cause of most cardiovascular disease and the leading cause of debilitating illness and death in this country. The PDAY findings emphasize the need to modify risk factors in young people to retard the development of atherosclerotic lesions, particularly clinically significant lesions. Thus, true primary prevention of atherosclerosis must being in childhood or early adolescence.