In a prospective study, forty one patients underwent myocardial imaging using 201 thallium (²⁰¹ Tl) scmtigraphy before and 6 months after coronary artery bypass grafting (CABG). The results were compared with the findings at coronary arleriography performed at the same time.²⁰¹ Tl was injected at peak exercise level performing an exercise scintigram. A thallium image equivalent to a redistribution scan was obtained at rest approximately 4 h after the injection of²⁰¹ Tl.The overall angiographic patency rate was 0.77. Postoperative myocardial perfusion was improved in 26 patients (88%) having 72 of 83 grafts patent (patency rate 0.87). New perfusion defects or unchanged ischaemic patterns were found in 5 patients, who had 2 of 13 grafts patent (graft patency 0.15). Estimation of the graft status by ²⁰¹ Tl-scinligraphy showed a sensitivity, specificity, and predictive value of a positive and negative result of 0.71, 0.94, 0.79 and 0.91, respectively, using angiographic findings as a reference.It is concluded, that comparison between preoperative and postoperative ²⁰¹ Tl imaging of the myocardium at exercise and rest provides useful information on graft patency after CABG. The procedure is superior to graft patency estimation based on resting or postoperative scintigrams only.