SUMMARYThe aim of this study was to assess the value of the left ventricular cavity-to-myocardium count ratio (C/M ratio) of technetium-99m (Tc-99m) tetrofosmin single photon emission computed tomography (SPECT) to identify abnormal left ventricular ejection fraction (LVEF) responses after exercise in patients with coronary artery diseases (CAD). We studied 50 patients with recent CAD undergoing rest and exercise first-pass ventriculography to calculate LVEF and rest and exercise Tc-99m tetrofosmin myocardial perfusion SPECT to calculate left ventricular C/M ratios. Group A, consisting of 25 CAD patients with normal responses (increased LVEF≥5% after exercise), had significantly higher rest and exercise C/M ratios than those of the group B, consisting of 25 CAD patients with abnormal responses (increased LVEF <5% after exercise) after exercise. However, the C/M ratios between exercise and rest did not differ significantly between groups A and B. In addition, there was significant correlation between LVEF and C/M ratios in all of the patients. C/M ratios of Tc-99m tetrofosmin myocardial perfusion SPECT are useful parameters for identifying patients with abnormal LVEF responses among patients with CAD. (Jpn Heart J 2002; 43: 505-514)