At present, it is widely assumed that the physiological significance of a coronary obstruction can be assessed by measuring percent stenosis on a coronary arteriogram. This assumption is being challenged by physiological measurements of coronary reserve performed in patients at open heart surgery with a Doppler technique. These physiological studies have shown a poor correlation between coronary reserve and percent stenosis. Although stenoses at the ends of the spectrum (less than 10% or greater than 90% diameter narrowing) have expected effects on coronary reserve, stenoses of intermediate severity (10-90%) do not accurately predict coronary reserve in the individual vessel involved. Studies with quantitative angiography indicate that diffuse coronary atherosclerosis, undetected by angiography, is probably the dominant explanation for why percent stenosis does not accurately predict the physiologic significance of coronary obstructions in man. Because the 'gold standard' (percent stenosis) utilized for decades to assess the physiologic significance of coronary obstructive lesions has been seriously challenged, development of other more sophisticated approaches to this problem should be encouraged.