Although internal mammary artery bypass grafts have a high patency rate, the adequacy of blood flow through such conduits, particularly if used sequentially, has been questioned. To evaluate this issue, coronary flow reserve was studied in 20 patients after coronary bypass surgery. Nine patients had sequential internal mammary grafts to the diagonal and left anterior descending coronary arteries; five had a single internal mammary graft to the left anterior descending artery and six had sequential saphenous vein grafts. Fifteen additional single vein grafts were also placed in these patients. Coronary flow reserve was measured after contrast-induced hyperemia by a digital subtraction angiographic technique an average of 25 days after surgery. There was no difference in coronary flow reserve between the proximal and distal anastomotic regions in either the sequential internal mammary graft group (2.14 +/- 0.50 versus 2.29 +/- 0.68, n = 8, p = NS) or the sequential vein group (1.77 +/- 0.49 versus 2.08 +/- 0.78, n = 6, p = NS). In addition, the flow reserve provided to either vascular bed of the sequential internal mammary graft was not different from that provided by a single internal mammary graft (1.64 +/- 0.39, n = 5), a single vein graft (1.95 +/- 0.95, n = 15) or nonstenotic native coronary arteries (2.04 +/- 0.87, n = 34). No cases of intracoronary steal were observed. Although some patients had unequal flow reserves between the proximal and distal anastomotic zones, these occurred in the setting of residual coronary stenoses distal to the site of graft insertion or prior myocardial infarction in the grafted distribution.(ABSTRACT TRUNCATED AT 250 WORDS)