The regional myocardial perfusion distribution of coronary artery bypass grafts were studied in 61 patients who received 162 grafts. Selective intragraft instillations of radioactive-labeled macroaggregated albumin particles were used to study perfusion. The extent of individual graft perfusion was assessed in 100 patent grafts. Regional myocardial blood flow distribution was similar to the blood flow distribution of the native vessel receiving the graft in 64 of 100 grafts and less than that expected of the native vessel in 12 grafts. However, 24 grafts demonstrated a blood flow distribution pattern which extended beyond the normal distribution expected of the native vessel receiving the graft. This extensive perfusion could be attributed to collateral vessels or retrograde flow. A high incidence of both graft and native vessel occlusion was found in areas receiving blood from these distant grafts with extensive distributions (16/24), and left ventricular wall motion was preserved or significantly improved postoperatively in 28/31 segments in such areas. When comparing angiographic and scintigraphic methods of evaluating myocardial perfusion, the angiogram underestimated the full extent of graft blood flow distribution in 13 of 24 instances (54 percent).