To investigate changes in the major components of atherosclerotic lesions during the progression of this disease, we measured the lesional areas of macrophages, smooth muscle cells, collagen fibers, and extracellular lipid deposits in the aortas of WHHL rabbits. Aortic segments with lesions of various stages were stained for histological and immunohistochemical examination, and the area of each lesional component was measured by a color image analyzer. In the early fatty streaks observed in 3-month-old rabbits, macrophages were predominant in the intima and were also observed in the inner layer of the media. In the transitional lesions (fibro-fatty streaks) found in rabbits at 11 to 15 months of age, an increase in the lesional area of macrophages was prominent compared to other lesional components. Thus, macrophages may play an important role in the progression of aortic atherosclerosis at this stage. In advanced complicated lesions observed in rabbits at 20 to 24 months of age, the area of macrophages and smooth muscle cells did not increase, whereas the area of collagen fibers and extracellular lipid deposits increased. Therefore, both the disruption of foam cells and fibrosis may play an important role in the progression of atherosclerosis at this stage.