The Cholesterol Lowering Atherosclerosis Study (CLAS) and the Monitored Atherosclerosis Regression Study (MARS) are serial arterial imaging clinical trials that have explored the reversibility of atherosclerosis with lipid-lowering therapy in native coronary, carotid, and femoral arterial beds, as well as in coronary artery bypass grafts. Results demonstrate that progression of atherosclerosis can be reduced in all these vascular beds. Evolving data indicate that coronary lesions > or = 50% diameter stenosis (%S) at baseline respond more readily to lipid-lowering therapy than lesions <50%S. In addition, lipoproteins may have a differential effect on coronary lesion progression according to lesion size, with triglyceride-rich lipoproteins playing an important role in the progression of lesions <50%S. Limited data indicate that progression of atherosclerosis in women may be more responsive to lipid-lowering therapy than in men, and that estrogen replacement may enhance the anti-atherosclerosis effects of lipid lowering. Longitudinal measurements of carotid artery far wall intima-media thickness (IMT) with B-mode ultrasonography in CLAS and MARS indicate that carotid atherosclerosis at a stage before lesions intrude into the vessel lumen can be reduced by lipid-lowering therapy. Together, CLAS and MARS data indicate that the spectrum from very early lesions confined to the arterial wall to established lesions late in the atherosclerotic process can be reversed with lipid-lowering therapy.