We previously generated transgenic mice expressing human apolipoprotein (apo-) B and demonstrated that the plasma of chow-fed transgenic animals contained markedly increased amounts of LDL (Linton, M. F., R. V. Farese, Jr., G. Chiesa, D. S. Grass, P. Chin, R. E. Hammer, H. H. Hobbs, and S. G. Young 1992. J. Clin. Invest. 92:3029-3037). In this study, we fed groups of transgenic and nontransgenic mice either a chow diet or a diet high in fat (16%) and cholesterol (1.25%). Lipid and lipoprotein levels were assessed, and after 18 wk of diet, the extent of aortic atherosclerotic lesions in each group of animals was quantified. Compared with the female transgenic mice on the chow diet, female transgenic mice on the high-fat diet had higher plasma levels of cholesterol (312 +/- 17 vs 144 +/- 7 mg/dl; P < 0.0001) and human apo-B (120 +/- 8 vs 84 +/- 3 mg/dl; P < 0.0001). The higher human apo-B levels were due to increased plasma levels of human apo-B48; the human apo-B100 levels did not differ in animals on the two diets. In mice on the high-fat diet, most of the human apo-B48 and apo-B100 was found in LDL-sized particles. Compared with nontransgenic mice on the high-fat diet, the transgenic animals on the high-fat diet had significantly increased levels of total cholesterol (312 +/- 17 vs 230 +/- 19 mg/dl; P < 0.0001) and non-HDL cholesterol (283 +/- 17 vs 193 +/- 19 mg/dl; P < 0.0001). The extent of atherosclerotic lesion development within the ascending aorta was quantified by measuring total lesion area in 60 progressive sections, using computer-assisted image analysis. Neither the chow-fed transgenic mice nor the chow-fed nontransgenic mice had significant atherosclerotic lesions. Nontransgenic animals on the high-fat diet had relatively small atherosclerotic lesions (< 15,000 microns 2/section), almost all of which were confined to the proximal 400 microns of the aorta near the aortic valve. In contrast, transgenic animals on the high-fat diet had extensive atherosclerotic lesions (> 160,000 microns 2/section) that were widely distributed throughout the proximal 1,200 microns of the aorta. Thus, human apo-B expression, in the setting of a diet rich in fats, causes severe atherosclerosis in mice.