Lung/heart uptake ratio (L/H R) in 201T1 myocardial perfusion scan is a reliable marker for long-term prognosis in patients with coronary artery disease. However, the value of L/H R in Tc-99m-MIBI myocardial perfusion scan is controversial in determining the prognosis and severity of the coronary artery disease. The purpose of this study was to determine the clinical implications of L/HR in Tc-99m-MIBI myocardial perfusion scan. Forty five patients who received Tc- 99m-MIBI myocardial perfusion scan were divided into control group and coronary artery disease (CAD) group by their clinical findings, EKGs, and Tc-99m-MIBI myocardial perfusion scans. Twenty five patients in CAD group were, divided into ischemicc group and infarct group according to their results from Tc-99m-MIBI myocardial perfusion scan. L/H R was calculated on the anterior planar view, 60 minutes after infusion of dipyridamole. Two regions of interest (ROI) were placed on the left lung area 8 pixel above the left ventricle and on the myocardial area which had the highest radioactivity. In the control group, there were no significant differences of L/H R according to sex and age. No significant difference of L/H R was found between the control and CAD group (0.26±0.06, 0.29±0.05, p>0.05). In the CAD group, there was also no significant difference of L/H R between the ischemic group and infarct group (0.29±0.07, 0.30±0.04, p〉 0.05). L/H R in CAD group did not show correlations with the defect are of stress polar map (r=0.18, p〉 0.05) and with the sum of severity weighted extent score or reversibility score which represent severity and extent of myocardial perfusion defect area in stress (r=0.18, p〉0.05). We conclude that it is difficult to use L/H R as a marker for severity of CAD in dipyridamole Tc-99m-MIBI myocardial perfusion scan.