The aim of this study was to establish a criterion for the success of an angioplasty based upon pressure gradients across coronary lesions. Sixty-two percutaneous transluminal coronary angioplasties (PTCA) in 56 patients with isolated left anterior descending artery disease were examined. Pressure gradients measured before and after PTCA were expressed as normalized mean pressure gradients (NMPG) computed by dividing mean pressure gradient by mean aortic or proximal coronary artery pressure. Angiographic severity was expressed as percentage area stenosis (AS) calculated from diastolic caliper measurements of diameter of each lesion and the nearest normal adjacent segment in at least two projections. The relationship between AS and NMPG was nonlinear with a steep increase in gradients beyond a critical value of AS of about 60%. This relationship was unaffected by angiographically visualized collaterals. All except one of 65 coronary stenotic lesions with NMPG of more than 0.32 had an AS of more than 60%. Only three of 57 coronary stenoses with NMPG of less than 0.32 had severe AS (p less than 0.001). The results indicate that NMPG is a reliable, practical guide to the severity of coronary stenosis and is therefore a useful measurement for assessing either the success or the residual stenosis during PTCA.