Angiography frequently demonstrates a collateral circulation in severe coronary artery disease. An easily applicable method to quantify collateral flow might be a useful adjunct for the assessment of the hemodynamic effects of coronary artery disease. The purpose of this study was to validate a visual scaling of the extent of angiographic collateral filling by comparison with flowmeter- and microsphere-derived measurements of collateral flow. In 12 open-chest dogs, collaterals from the circumflex artery were angiographically visualized (n = 80) by creating acute critical left anterior descending artery occlusion. The extent of collateral filling was graded in four levels from 0 = no visible filling to 3 = complete epicardial filling. Collateral filling correlated with the change in flow of the collateral supplying circumflex artery (delta Q; r = 0.84) which was + 5.3 +/- 4.6% with grade 1, + 9.1 +/- 3.5% with grade 2 and + 14.6 +/- 4.7% with grade 3 (p < 0.01). In parallel, coronary flow reserve decreased from 4.1 +/- 0.8 with grade 0 to 2.9 +/- 0.2 with grade 3 (p < 0.01). Colored microspheres were injected subselectively into the circumflex artery of 9 dogs (45 injections). The ratio of microspheres counted in the collateralized myocardium of the left anterior descending artery to the total number injected increased from 0.6 +/- 0.9% for grade 0 to 17.1 +/- 2.8% with grade 3 (p < 0.01). Absolute collateral flow derived from the microsphere counts averaged 5.5 +/- 0.9 ml/min with grade 3 and closely correlated with collateral filling grade (r = 0.88). Semiquantitative grading of angiographic collateral filling in response to acute coronary occlusion in a canine model correlates with an increase in collateral source artery flow, absolute collateral flow and a decrease in source artery flow reserve. These data suggest that this scale might be a simple but useful adjunct tool to assess the hemodynamic significance of a collateral circulation.