Aim of this study is the assessment o? feasibility and clinical usefulness of a new index of stenosis severity, the slope of the instantaneous transstenotic pressure gradient/ velocity relationship. Twenty-one patients scheduled for percutaneous revascularization procedures were studied wlth simultaneous measurement of poststenotic coronary pressure and flow veloclty, in basal condition and during maximal hyperemia induced with intracoronary papaverlne. Reliable measurements of the transstenotic pressure gradient/ velocity relationship could be obtained in 11 patients. In 64% of the cases, a quadratic equation showed the best fit for the data. Steeper increases of the transstenotic pressure gradient at any given velocity increase were observed in the lesions with the smallest cross-sectional area measured wlth quantitative angiography. A comparlson of this new index wlth coronary flow reserve, maximal hyperemic velocity, stenosis ?low reserve derived from quantitative angiography, basal and hyperemic transstenotic pressure gradient and fractional flow reserve is presented and the relative merits of all these parameters are discussed. This pilot experience suggests that the instantaneous relationship between pressure gradient and flow velocity changes during the cardiac cycle can accurately characterize the stenosis hemodynamics in the catheterizatlon laboratory.