Fifty patients with left main coronary artery disease were studied to evaluate the functional role of collateral circulation. The left main 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 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.