Intra-aortic balloon counterpulsation is capable of reducing afterload in patients with unstable angina. Whether it is also capable of augmenting coronary blood flow to poststenotic myocardium is controversial. We studied seven patients receiving maximal drug therapy and requiring balloon pumping for unstable angina as balloon volume and assist ratio were altered. All patients had greater than 90% stenosis of the proximal left anterior descending coronary artery. With maximal augmentation (40 cc balloon volume, 1:1 assist ratio) great cardiac vein flow, representing the efflux from the left anterior descending coronary artery bed, rose from a baseline of 52 +/- 20 to 67 +/- 25 ml/min (mean +/- SD, p = .004) and mean aortic diastolic pressure increased from 77 +/- 13 to 99 +/- 33 mm Hg (p = .004). Increased great cardiac vein flow correlated with increased mean aortic diastolic pressure across changes in balloon volumes (off, 20 cc, 30 cc, and 40 cc) and changes in assist ratio (off, 1:4, 1:2, and 1:1) (p = .02). However, the intermediate balloon volumes produced great cardiac vein flows at an intermediate level between full assist and no assist (p less than .05), whereas the intermediate assist ratios did not augment flow. Thus balloon pumping increased flow to a bed fed by collateral vessels or critical stenoses; this increased flow correlated with increased aortic diastolic pressure, indicating probable loss of autoregulatory ability.