Effects of coronary angioplasty on myocardial flow reserve have been difficult to characterize noninvasively because conventional imaging techniques cannot quantitate blood flow in absolute terms. The effects of coronary angioplasty on myocardial perfusion and perfusion reserve were delineated with positron emission tomography and oxygen-15-labeled water (H215O) in 13 patients before and after single vessel angioplasty. In 11 patients, angioplasty was successful (minimal cross-sectional area increased from 0.60 ± 0.59 to 3.45 ± 1.09 mm2, p < 0.001). In these patients, regional H215O radioactivity (the ratio of nutritional perfusion in regions distal to the stenosis compared with regions supplied by angiographically normal arteries) at rest before angioplasty was 55 ± 22% of peak myocardial radioactivity and did not increase significantly afterward (70 ± 16%, p = NS). However, after administration of intravenous dipyridamole, hyperemic perfusion in regions distal to a stenosis averaged only 39 ± 18% of peak myocardial counts before angioplasty, but increased to 66 ± 22% after angioplasty (p < 0.02). Perfusion reserve in the two patients in whom angioplasty was angiographically unsuccessful showed no change. Quantitative estimates of perfusion in absolute rather than relative terms were obtained with positron emission tomographic data from seven of the patients with successful angioplasty. At rest, perfusion in regions distal to a stenosis was not different from the values in regions supplied by normal coronary arteries (1.54 ± 0.54 compared with 1.46 ± 0.38 ml/g per min, p = NS). After dipyridamole, however, perfusion distal to a stenosis increased to only 64 ± 14% of flow observed in normal regions (1.98 ± 0.97 compared with 3.24 ± 1.92 ml/g per min, p < 0.04). In contrast, after angioplasty, hyperemic perfusion unproved in regions supplied by vessels subjected to angioplasty with values similar to those in regions supplied by normal vessels (4.36 ± 1.61 compared with 4.04 ± 1.28 ml/g per mm). Eight patients, all without symptoms of recurrent ischemia, were studied again an average of 7.5 months after angioplasty. In all, the enhanced perfusion reserve capacity was maintained. The results obtained indicate that angiographically successful coronary angioplasty improves myocardial perfusion reserve and demonstrate the utility of positron emission tomography with H215O for quantitative delineation of the efficacy of specific means of revascularization on nutritional perfusion and perfusion reserve.