Accelerated progression of atherosclerosis is known to occur in surgically bypassed coronary arteries in which the preoperative stenosis was greater than 50 per cent. To assess the effect of coronary bypass on vessels with lesser degrees of stenosis, we studied 85 men who had undergone coronary bypass surgery. In this group we identified bypass grafts placed in 37 arteries with minimal atherosclerosis, which was defined as less than 50 per cent stenosis of the vessel diameter. In the same 85 men there were 93 coronary vessels with minimal atherosclerosis for which a bypass graft had not been placed. Progression of atherosclerosis, defined as further loss of at least 25 per cent of the lumen, during an average follow-up period of 37 months was more than 10 times as frequent (38 per cent vs. 3 per cent) in bypassed arteries with minimal atherosclerosis as in comparable arteries that were not bypassed. These findings support the view that minimally diseased coronary arteries should not be bypassed.