To determine the clinical significance of intimal proliferation as a cause of aortocoronary bypass vein graft obstruction, 492 vein grafts from 281 patients were studied 0 to 75 months postoperatively. All grafts had been inserted between 1968 and 1975 by one surgeon using one technique. The graft patency rate was 92 percent (55 to 60) in the first month; 91 percent (49 of 54) at 1 to 3 months; 84 percent (37 to 44) at 4 to 6 months; 77 percent (33 of 43) at 7 to 12 months; 81 percent (113 of 140) at 13 to 24 months; 82 percent (59 of 72) at 25 to 36 months; and 84 percent (66 of 79) at 37 to 75 months. Vein graft samples were obtained from 41 patients: In 27 patients with 51 grafts (early group), they were obtained 0 to 30 days (mean 14 days) postoperatively; in 14 patients with 27 grafts (late group) they were obtained 7 to 75 months (mean 34 months) postoperatively. Intimal proliferation was graded 1 to 4 corresponding to an intima/media thickness ratio of 1,2,3 or 4, respectively. In the early group, all 51 vein grafts howed grade 1 to 2 intimal proliferation; 5 of these grafts were occluded, all as a result of recent thrombosis. In the late group, 17 of the 27 grafts were studied histologically. All patent vein grafts showed grade 2 to 3 intimal prolferation. Four vein grafts were occluded but only one as a result of grade 4 intimal proliferation. In 14 patients in the late group, angiograms performed shortly before vein graft samples were obtained revealed 14 patent and 4 occluded vein grafts. Ten of the 14 patent vein grafts showed grade 2 to 3 intimal proliferation but were of uniformly good caliber angiographically (graft/artery ratio more than 1.5).