There has been clinical evidence that a perfusion defect on a stress image fills in over time. The diagnostic value of initial and 120 min post exercise redistribution thallium-201 myocardial images (RMI) was determined in 120 pts. with suspected coronary heart disease (CAD), all of whom had coronary arteriography. Significant (greater than or equal to 75%) lesions were present in 88 pts. 30 pts. without CAD showed a normal tracer uptake immediately after exercise. Scintigrams taken 120 min after exercise revealed a decrease of 201-Tl concentration in every area of the myocardium. 80 pts. with CAD showed an area of decreased tracer uptake in the initial scans. 120 min RMI in 51 pts. revealed a segnificant increase (p greater than 0.01) of countrate time ratio in previous underperfused area. In 37 pts. persistent defects were present, in every case the defect correlated with the site of a myocardial infarction as determined by the finding of an akinetic area in the left ventricular angiogram. Thus RMI following a single dose of 201 Tl can differentiate between scar- and exercise-induced transient ischemia.