A fibreoptic pressure sensor mounted on an 0.018 inch guidewire (Pressure Guide, RadiMedical Systems, Uppsala, Sweden) was used to measure the trans-stenotic pressure gradient in 20 patients admitted for percutaneous transluminal coronary angioplasty (PTCA) of a single, discrete stenosis. Pressure measurements were made both at rest and during maximal vasodilatation induced by intracoronary injection of papaverine. From the ratio of distal coronary pressure divided by the proximal pressure, the relative coronary flow reserve was calculated. The aim of the study was to compare the different pressure-derived parameters by correlating them to stenosis geometry estimated by quantitative coronary angiography. There was a moderate correlation between baseline pressure gradient and percent area stenosis; r = 0.64, P < 0.001 and minimal cross-sectional area; r = 0.45, P < 0.005. A higher correlation was found between hyperaemic pressure gradient and area stenosis (r = 0.80, P < 0.001) and minimal cross-sectional areas, respectively (r = 0.55, P < 0.005). The best correlation was found between relative coronary flow reserve and area stenosis (r = 0.86, P < 0.001) and minimal cross-sectional area (r = 0.70, P < 0.001). In conclusion, pressure measurement using a pressure guidewire is useful as a complement to angiography in evaluation of coronary stenoses during PTCA. Pressures should be measured during maximal vasodilatation. Relative coronary flow reserve calculated from the pressure measurements provides additional information about the fraction of normal maximal flow possible in the presence of a stenosis.