In the presence of obstructing coronary atherosclerosis, collateral circulation may develop through pre-existing anastomotic channels which are present, though nonfunctional, in normal hearts. These anastomoses connect various branches of the same coronary artery or different coronary arteries. The common collateral pathways found among a group of 200 patients with significant coronary artery disease are described. Collateral circulation is only visible angiographically when severe narrowing (greater than 90 per cent) or total occlusion of the coronary artery is present.