Coronary angiography should be performed to verify the patency of bypass graft if coronary artery disease is said to be surgically corrected. Rest/stress myocardial SPECT could be used as a noninvasive alternative to prove patency of bypass graft and functional significance of graft patency by showing improvement of rest perfusion and perfusion reserve after operation. We evaluated the outcome of coronary artery bypass graft surgery by comparing rest/dipyridamole-stress myocardial SPECT before and after operation. Myocardial SPECT were performed $19{pm}23$ days before and $108{pm}19$ days after operation in 44 patients(M:F=25:19). Segmental perfusion were scored with 0(normal) to 3(defect) independently for each image set of rest and stress. Perfusion scores were compared between pre and postoperative images. Rest/stress images of pre and post-operative SPECT were compared in paired sets. Postoperative change of perfusion was determined as no change, improvement or aggravation for each artery territory. Postoperative outcome of patients were analyzed. Overall, 74%(158 segments) of 215 segments which had perfusion decrease before operation showed improvement. Among 60 artery territories, 37 territories(62%) improved. Among 42 patients with perfusion decrease in preoperative SPECT, 21 patients(47%) improved after operation. Severe persistent defects improved in 43%(6/14). We concluded that bypass surgery improved myocardial perfusion in dipyridamole stress induced or persistent decrease and that rest/stress myocardial SPECT could be used as a guide for which artery should be operated.