The degree of arteriographically visualized narrowing in coronary arteries has been the most important criterion for evaluating coronary disease for more than 20 years. Increasing data, however, suggest that anatomy alone does not predict the physiologic consequence of individual stenoses. We have applied a new digital arteriographic method for measuring coronary flow reserve (CFR) in patients undergoing diagnostic, postbypass, and postangioplasty catheterizations. All vessels with high-grade stenoses (greater than or equal to 70%) were found to have lower CFRs than those of normal arteries (1.01 +/- 0.15 v 1.84 +/- 0.36). However, considerable CFR variability was found in vessels of lesser stenoses. Coronary arteries that had undergone successful bypass surgery or angioplasty had intermediate CFR levels (1.44 +/- 0.18 and 1.51 +/- 0.16, respectively). Our study suggests that CFR assessed during cardiac catheterization may be useful for evaluating intermediate coronary lesions and the efficacy of interventions.