The intracellular distribution of lipid shifts from cytoplasmic to lysosomal localization during the progression of atherosclerosis. It has been suggested that this shift may relate to regressability of lesions. The effects of reducing plasma cholesterol on the regression of early cholesterol-induced atherosclerosis were evaluated. Most small, early lesions disappeared after 5 weeks on the regression regimen. Larger lesions, however, did not change in extent even following 10 weeks regression. Although large lesions were not reduced in size under the regression conditions, total cytoplasmic lipid decreased. Paradoxically, the size of residual intracellular lipid deposits increased. The structural features of these remaining deposits suggest that they were lipid-filled lysosomes. Leukocyte adherence to endothelium, which increases 10- to 20-fold during progression, returned to control levels over most areas of large lesions. Levels of adherent leukocytes remained elevated, however, over small lesions and at the edges of larger lesions. Our data indicate that regression is not a uniform process, but rather, even in early lesions, varies within separate intimal microdomains. In addition, our data suggest that part of the difference may reside in differential partitioning of lipid into lysosomes.