A prospective angiographic study of the coronary collateral circulation was performed in 600 consecutive patients found to have significant coronary arterial disease at cardiac catheterization. As the number of diseased coronary vessels increased the evidence of collaterals rose significantly (P = 0.00002) and the intensity of the collateral circulation increased (P = 0.05). For totally occluded lesions the collateral incidence was 70.4%, independent of the particular vessel involved. For stenoses greater than 90% collateral frequency and intensity were significantly greater for lesions of the right coronary artery than of the left anterior descending and the left circumflex arteries. The distribution of the coronary collateral circulation is described quantitatively according to the site of the lesion. Finally, a coronary collateral circulation occurred more frequently (P less than 0.000 001) and excepting Grade I, the intensity of the coronary collateral circulation was significantly greater in patients with a history of acute myocardial infarction.