ISUAL ASSESSMENT of the diameter percentage of stenosis of a diseased coronary artery still remains the “gold standard” in more than 99% of hospitals that perform coronary angiography.’ Several experimental studies have shown that percent diameter stenosis, under carefully controlled conditions, provides a useful index of the significance of a coronary lesion.*-” However, the general applicability of this index to clinical coronary disease, especially when it is diffuse and affects more than one vessel, has been seriously questioned.‘*” Furthermore, subjective assessment of the extent of a coronary obstruction has been found to correlate poorly with pathoanatomic findingP’ and to suffer significant interobserver and intraobserver variability.2”-28 With the appearance of more accurate, objective methods for the assessment of stenotic dimensions,‘9-3’ it became possible to overcome many of the limitations of the subjective visual approximation of percent diameter stenosis alone. Visual assessment can be avoided with the use of computer programs that analyze the coronary lesion either by automated borderdetection of the affected vessel29 or by densitometric analysis that makes use of the concept that, in an ideal setting, the density across an artery in a radiographic image is proportional to the cross-sectional area at that point.“’ In addition, other indices of the stenosis severity such as the absolute cross-sectional area of the stenosis have been proposed32 and can be now easily and reproducibly obtained. This anatomic/ geometric assessment of coronary stenosis severity has been further expanded by the development of more comprehensive methods that take into account all the dimensions of the lesion as measured on coronary angiograms.33 Furthermore, several methods for the assessment of the physiological significance of the stenosis by measuring the coronary flow reserve have been developed.34”5 These techniques are based on actual measurements of coronary flow or evaluation of myocardial perfusion, thus comprising a functional/physiological approach to assessing the impact of the lesion on the coronary circulation. All these indices with their limitations, have been used as single measures of the stenosis severity. Their value to predict the haemodynamic significance of any lesion has been examined and this review will analyze in detail the theoretic and experimental data describing these different indices of stenosis severity.