In the last ten years the internal mammary artery (IMA) has, thanks to its higher patency rate, been increasingly used for revascularization of the coronary arteries in preference to the saphenous vein. However, uncertainty about atherosclerosis in the IMA has continued to be voiced, and to clarify the issue we investigated post mortem the IMA and left anterior descending (LAD) coronary artery in 48 male patients who died aged 45-75 years. Six segments of the IMA and 4 of the LAD, totalling 465 histologic cross sections, were estimated quantitatively. The result was unequivocal: the tendency to stenosing intimal sclerosis is much more marked in the coronary artery than in the IMA. The IMA is not immune to intimal thickening, but even in patients with severe coronary artery disease the degree of atherosclerosis in the IMA is not of functional relevance. This fact makes the IMA the vessel of choice for coronary bypass surgery.