The potential benefit of platelet inhibitor drugs on coronary arterial bypass vein grafts was assessed in dogs with magnification-corrected angiographic luminal measurements and quantitative histologic evaluation of the vein grafts. There were 11 control animals and 11 animals treated with dipyridamole, 55 mg/day, plus aspirin, 325 mg/day. Eighteen animals with patent grafts were studied when electively killed 2, 4 or 6 months after grafting. At 14 days, there was greater angiographic narrowing in the most distal 1 cm of vein grafts in control than in treated dogs (P less than 0.01). This same angiographic narrowing persisted in control dogs until they were killed (P less than 0.03). Computer-assisted measurements of the entire area of intimal thickening were done on vein graft cross sections taken 1 cm from the distal anastomosis. The circumference of the vein grafts at the intimal-media junction was measured from the same section and the potential maximal luminal area calculated. The calculated luminal narrowing due to intimal thickening was greater in control than in treated dogs (P less than 0.03). These data correlate well with the demonstrated angiographic narrowing. The findings indicate that the degree of early intimal thickening that persists 2 to 6 months postoperatively in canine coronary bypass vein grafts may be reduced by the platelet inhibitor combination of dipyridamole plus aspirin.