The quantification of aortic lesions is an important end-point analysis for evaluating atherogenesis in mouse models of atherosclerosis. Morphometric methods involving the staining of aorta with a Sudan lysochrome followed by image analysis of the stained lesion area are commonly used. We have developed a more rapid method involving solubilisation of the stain retained by aortic lesions. In 2 separate studies, 5-week-old male apoE<sup>–/–</sup> and C57BL/6 wild-type (apoE<sup>+/+</sup>) mice were given a high fat (21%), Western-type diet for 13, 15 or 25 weeks. At study termination, the descending thoracic aorta (DA) and/or aortic arch (AA) were stained with Oil Red O (ORO). The incorporated stain was extracted using chloroform/methanol (2:1) solvent and quantified by spectrophotometry at 520 nm. In study 1 (13 weeks), ORO stain in the AA and DA of apoE<sup>–/–</sup> mice was 1.9 and 1.4 times higher than background staining of apoE<sup>+/+</sup> aorta tissue, respectively. At 15 and 25 weeks (study 2), ORO stain in the AA of apoE<sup>–/–</sup> mice was 1.9 and 2.5 times higher than the background, respectively. We conclude that the ORO solubilisation technique applied to AA samples is a very useful and rapid method for atherosclerotic lesion quantification.