The clinical role of collateral vessels was evaluated during transient coronary occlusion by percutaneous transluminal coronary angioplasty in 22 patients with (8) and without (14) collateral vessels. Coronary occlusion pressure, the ratio of mean coronary occlusion pressure to mean aortic pressure and myocardial perfusion pressure at 40 s of balloon inflation were significantly higher in patients with than in patients without collateral vessels. The changes in left ventricular systolic and end-diastolic pressure, maximal rate of rise of left ventricular pressure (peak dP/dt) and maximal rate of fall of left ventricular pressure (negative peak dP/dt) during balloon inflation were less in patients with than in patients without collateral vessels. Myocardial lactate was produced in patients without collateral vessels but not in those with such vessels. Marked ST segment elevation in the electrocardiogram occurred in patients without collateral vessels but either ST segment depression or mild ST segment elevation was observed in patients with collateral vessels. This study indicates that collateral vessels limit myocardial ischemia during coronary occlusion, probably as a result of increased myocardial perfusion pressure.