Quantitation of coronary luminal diameter with a 20 MHz mechanically rotating intravascular ultrasound (IVUS) catheter was compared with orthogonal-view cineangiography by use of a semiautomated edge-detection algorithm in 48 patients undergoing coronary angioplasty. Quantitative comparison of 196 matched segments was attempted, but in only 174 (88.8%) was a direct comparison of the two techniques possible. In angiographically normal coronary arteries (46 segments) the correlation between the values obtained by quantitative coronary angiography (QCA) and those achieved by IVUS was excellent (r = 0.92, p < 0.0001). For mild stenoses (80 segments) the correlation coefficient was only fair (r = 0.467, p < 0.001). After percutaneous transluminal coronary angioplasty the correlation coefficient between IVUS and QCA data (48 segments) was very weak (r = 0.282, p < 0.05). In conclusion, coronary IVUS is feasible and safe and even for a limited range of coronary arterial narrowing, significant correlations between IVUS and QCA measurements of minimal lumen diameter were found. They were excellent in normal coronary arteries, moderate in mildly diseased arteries, and weak after balloon angioplasty.