To evaluate the fate of the boronary arteries after aortocoronary bypass, 40 patients underwent serial selective coronary angiographic studies 1 year apart, and the frequency of progression of coronary artery disease was estimated. Thirty-two had saphenous vein bypass surgery after the first procedure, six had Vineberg operations, and two had no interim operation. In each patient, the right, left, anterior descending and circumflex coronary arteries (including their branches) were separately evaluated. Progressive narrowing was evident in 31 of 50 (62 per cent) bypassed vessels and in only 11 to 113 (9.7 per cent) nonbypassed arteries (p less than 0.001). Coronary arteries with moderate to severe obstruction initially (50 to 99 per cent occluded) manifested progressive diseasee more frequently (33 of 70 arteries) than did arteries that were normal or mildly narrowed initially (4 of 71) (p less than 0.001. Considering only those vessels with 50 to 99 per cent obstruction initially, 27 of 35 (77 per cent) of the bypassed arteries and only 6 of 35 (17 per cent) of the nonbypassed arteries showed progression (p less than 0.001). We conclude that moderately or severely narrowed coronary arteries are more likely to show progressive narrowing than normal or mildly obstructed ones and that progression of coronary disease is greater in bypassed vessels than in nonbypassed vessels. In view of potential graft closure, the implications of these findings must be considered in selecting patients for aortocoronary bypass.