To assess myocardial perfusion intraoperatively and to evaluate the adequacy of coronary bypass grafting, we measured regional myocardial blood flow by the electrolytic hydrogen clearance method in 49 patients. Group I comprised 10 patients with nonischemic heart disease and group II, 39 patients with ischemic heart disease undergoing coronary bypass grafting. Group II was subdivided according to the percent stenosis of the coronary arteries supplying the ventricular regions: group IIa, less than 75% stenosis; group IIb, greater than or equal to 75% stenosis. Mean myocardial blood flows were 154 +/- 7, 145 +/- 5, and 98 +/- 9 ml/min/100 gm in groups I, IIa, and IIb, respectively (p less than 0.01, group IIb versus groups I and IIa). Mean blood flows were 161 +/- 19, 159 +/- 12, 78 +/- 12, and 59 +/- 15 ml/min/100 gm in areas of the left anterior descending coronary artery with less than 50%, 75%, 90%, and 99% stenosis in group II. In areas with a totally occluded left anterior descending coronary artery with collaterals, mean flow was 90 +/- 15 ml/min/100 gm. The mean myocardial blood flows were 40 +/- 7 and 100 +/- 14 ml/min/100 gm in areas with anterior Q wave and non-Q wave infarction, respectively (p less than 0.01). After cardiopulmonary bypass, the mean flow increased from 99 +/- 11 to 150 +/- 7 ml/min/100 gm in the grafted areas in group IIb (p less than 0.01), but it did not change in group I or IIa. The electrolytic hydrogen clearance method provided quantitative evaluation of myocardial perfusion and recovery from hypoperfusion by coronary bypass grafting. This method was especially useful in patients undergoing mammary artery grafting.