Comparison was made between the morphologic condition of the left anterior descending artery and four arterial conduits: the internal mammary, right gastroepiploic, inferior epigastric, and radial arteries, harvested from 17 patients (aged 15 to 85 years, mean 64 years) who had died of nonvascular diseases. Proximal, mid, and distal segments were examined microscopically. The internal mammary artery was elastic, but the others were muscular. In all four conduits, atherosclerosis was absent to mild, the internal elastic lamina showed only minimal defects, and the vasa vasorum were confined to the adventitia. In all cases the left anterior descending artery showed mild to severe atherosclerosis and substantial defects in the internal elastic lamina with penetration of the vasa vasorum into the media and intima. Comparison of the mean distance (+/- standard deviation) from the lumen to the outermost portion of the media for the left anterior descending artery (320 +/- 63 microns) with the four conduits gave comparable values for the internal mammary artery (350 +/- 92 microns); p = not significant) and the right gastroepiploic artery (291 +/- 109 microns; p = not significant), versus 529 +/- 52 microns; p less than 0.002) for the radial artery and 249 microns (+/- 87 microns) (p less than 0.04) for the inferior epigastric artery (Kruskal-Wallis and Mann-Whitney U tests). The relatively scanty presence of smooth muscle cells in the thin-walled media of the internal mammary artery combined with a well-formed internal elastic lamina, even at advanced age, may be an important cause for its low susceptibility to atherosclerosis and a major determinant in its superior long-term patency as a coronary artery bypass graft. This finding emphasizes the justification of continued use of the ideally matching internal mammary artery, either as in situ or free graft, in coronary artery bypass grafting. In contrast to the thick-walled radial artery, which may be relatively prone to ischemia, an acceptable long-term patency of the inferior epigastric artery and right gastroepiploic artery, if harvested as pedicled grafts, is anticipated.