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To investigate the relation of the gradient across a coronary artery stenosis and the pressure distal to the stenosis after proximal occlusion during percutaneous transluminal coronary angioplasty to the amount of angiographically estimated collateral circulation, 63 patients (55 men, 8 women) were studied. All patients had 1-vessel disease (54 left anterior descending, 8 right coronary artery and 1 circumflex coronary artery). All patients had documented ischemia, and angioplasty was carried out within 4 weeks after the initial angiogram. The patients were separated into 4 groups: 0 = no collaterals (35 patients), +1 = just visible collaterals (8 patients), +2 = collaterals without reaching the contralateral vessel (10 patients), and +3 = filling of the contralateral vessel (10 patients). There was no difference in age among the 4 groups. There was a significant negative relation of the gradient vs the extent of collateral circulation, although the degree of stenosis increased significantly from group 0 to group +3. There was a significant positive relation of the occlusion pressure (in absolute terms and in percent of the proximal systolic pressure) vs the extent of collateral circulation. There was a significantly smaller change of the occlusion pressure vs the distal pressure before occlusion if good collaterals were present. The occlusion pressure remained constant during 1 occlusion up to 40 seconds and was reproducible in 3 successive occlusions. In conclusion, the pressure distal to a coronary artery stenosis is mainly dependent on the severity of the stenosis and on the collateral flow. If anterograde flow is eliminated by proximal occlusion the distal pressure is only dependent on the extent of collateral circulation.