Fifteen patients with significant coronary artery disease confirmed by cardiac catheterisation were studied before and after aortocoronary artery bypass graft surgery by rest and exercise 201Tl myocardial scintigraphy in an attempt to predict graft status non-invasively. Segmental myocardial perfusion comparisons in pre- and postoperative exercise images allowed correct predictions of high (greater than or equal to 67%) or low (less than or equal to 50%) graft patency rates in 10 of 15 patients, while similar comparisons in postoperative rest and exercise images yielded 13 of 15 correct predictions. Furthermore, 20 of 21 myocardial segments with increased postoperative perfusion when compared with corresponding segments in preoperative exercise studies were supplied by patent grafts. Regional graft occlusion, however, was difficult to predict. It was also difficult to predict graft status when myocardial segments were shown to have unchanged perfusion postoperatively as compared with the preoperative segmental assessment. Despite these limitations, the non-invasive technique of 201Tl imaging can provide useful information regarding coronary artery bypass graft patency.