In an autopsy study of the evolution of atherosclerotic lesions in young people, we obtained the coronary arteries and aortas of 1160 male and female subjects who died between full-term birth and age 29 years. In this article, we report the light and electron microscopic observations of the coronary arteries of 565 of these subjects in which we fixed the coronary arteries by perfusion with glutaraldehyde under pressure. From birth, the intima was always thicker in the half of the coronary artery circumference opposite the flow-divider wall of a bifurcation (eccentric thickening). In cases where we found lipid in the intima, there was always more in eccentric thickening. Isolated macrophage foam cells in the intima of infants were the earliest sign of lipid retention. These cells occurred in 45% of infants in the first 8 months of life but decreased subsequently. At puberty, more substantial accumulations of macrophage foam cells reappeared in more children. Foam cells were now accompanied by lipid droplets in existing smooth muscle cells and by thinly scattered extracellular lipid. Sixty-five percent of children between ages 12 and 14 years had such lesions. An additional 8% of children had progressed beyond this early stage and had developed advanced preatheroma or atheroma stages. Such advanced lesions, located only in areas of eccentric thickening, were characterized by the addition of massive extracellular lipid that displaced normal cells and matrix and, thus, damaged and weakened the arterial wall.(ABSTRACT TRUNCATED AT 250 WORDS)