Aortocoronary vein grafts are known to develop atherosclerotic plaques usually superimposed on intimal hyperplasia. The cellular characteristics of these lesions have been studied with immune cytochemical techniques and compared with those in native coronary arteries. Fifteen stenosed grafts showed concentric intimal hyperplasia characterized by massive proliferation of smooth muscle cells (HHF-35+). The subendothelial layer contained numerous T lymphocytes (UCHL-1+, MT-1+) and to a lesser extent macrophages (HAM-56+). Eleven grafts had superimposed atherosclerotic plaques characterized by atheroma but otherwise showing the same cellular constituents. The atherosclerotic plaques in the venous grafts resembled those in the coronary arteries, the main difference being the occurrence of multiple atheromas (up to 4 in a single section), the high number of T lymphocytes and macrophages related to these sites and the presence of atheromas bordering directly onto the luminal surface. It thus appears that the development of atherosclerotic plaques in vein grafts is accompanied by a similar immune inflammatory reaction as in native coronary atherosclerosis, presumably in a more aggravated form. The latter phenomenon could relate to the more enhanced and rapidly progressive nature of vein graft atherosclerosis.