Of 336 isolated coronary artery bypass procedures performed over an 11 month period, one internal mammary artery (IMA) was used in 81% of cases. In 41 patients, two IMAs were used for grafting three or more arterial segments in single or sequential fashion. Repeat coronary arteriography was performed in 26 of these patients 7 to 10 days after bypass surgery. The left IMA in situ was used either singly or sequentially to graft the left anterior descending (LAD) and diagonal arteries and all branches of the circumflex system. The right IMA was used to graft the LAD, diagonal, mid marginal, right, and posterior descending branch of the right coronary artery. Blood flow to a grafted arterial segment occurred primarily through the IMA in situ in 61% of segments bypassed but was equally distributed through the IMA graft and native circulation in 35%. This was in contradistinction to the free IMA and saphenous vein graft, in which the majority of flow was via the graft in 86% and 94% of grafted segments, respectively. For those instances in which major flow was via the IMA graft in situ, the mean diameter reduction of the proximally grafted arterial segment was 83% vs 63% for those segments in which flow was equally shared between native coronary and IMA graft (p less than .05). There was a trend, although not statistically significant, for increasing flow to occur via the native circulation if the first anastomosis in a sequential IMA graft in situ was performed to a larger artery than the distally grafted segment. This was especially true when there was low-grade stenosis in the proximally grafted arterial segment.(ABSTRACT TRUNCATED AT 250 WORDS)