The well-defined genetic systems of the mouse are proving useful in experimental studies of atherosclerosis. Inbred mouse strains differ in atherosclerosis susceptibility, and several variants of apolipoproteins have been identified and mapped. This report explores the location and timing of lesion formation in the mouse in an effort to provide a basis for quantitatively comparing groups of mice. After 14 weeks on an atherogenic diet containing 1.25% cholesterol, 15% fat, and 0.5% cholic acid, C57BL/6J female mice had aortic lesions at each of the intercostal arteries, at the junction of the aorta to the heart, and in scattered areas covering 1.1% +/- 0.5 (SD) of the aortic surface. After 9 months on the atherogenic diet, those lesions near the heart and intercostal arteries were extensive, 8% +/- 3 (SD) of the remainder of the aorta was involved in lesions, and lesions were found in the coronary arteries. Results indicated that one suitable location for scoring lesions was in a 300 micron area of the aorta just beyond the aortic sinus. The mean number of lesions/mouse in the selected area after 14 weeks on the atherogenic diet was 1.1 +/- 0.3 (SD). The results were reproducible over 10 separate experiments. The number of lesions per mouse fit a Poisson distribution indicating that the presence of one lesion did not predispose the mouse to acquiring a second lesion. Lesion formation and cholesterol levels did not vary with the season of the year as demonstrated by 9 separate experiments over more than 12 months. Methods of evaluating the number and size of lesions were compared including sizing with a microscope eyepiece grid and computer-assisted planimetry. The resulting data provide reproducible methods of quantitatively comparing lesion formation in various strains or groups of mice, thereby increasing the usefulness of the mouse as an experimental system for atherosclerosis research.