Fifty patients with left main coronary artery disease were studied to evaluate the functional role of collateral circulation. The left main coronary artery was narrowed 50-70% in 22 patients (group I), and more than 70% in 28 patients (group II). Significant disease in the other vessels was equally common in each group. There was no significant difference in the incidence of inter- and intracoronary collaterals in the two groups. Fifteen patients with no collaterals were compared with 35 patients with collaterals, and to a subset of 11 patients with very rich right-to-left collaterals, and there was no significant difference in historic or ECG evidence of old infarction, duration of angina, incidence of difference in historic or ECG evidence of old infarction, duration of angina, incidence of unstable angina, left ventricular end-diastolic pressure, cardiac index, ejection fraction, or segmental contraction abnormalities. We conclude that there is no evidence of protective effect of collateral vessels in patients with left main disease.