The relationship between the severity of coronary artery stenosis and the time to completed redistribution of thallium 201 (TI-201) defects following maximal exercise was investigated in 59 patients undergoing stress-redistribution TI-201 scintigraphy, coronary angiography, and contrast ventriculography. Multiple view TI-201 scintigrams were obtained, beginning 6 minutes (immediately post stress), less than 1 hour (early), 3 to 5 hours (average), and 18 to 24 hours (late) following intravenous TI-201 injection at peak exercise. Angiographic lesions were grouped into five levels of severity by percent stenosis. In the 107 defects which were seen on the immediate post stress images, early redistribution was noted in 15 (14%) and late redistribution was found in 23 (21%). In addition, there was a correlation (r = 0.56) between the time to completed redistribution and the severity of the coronary artery stenosis (p = 0.001). In comparison to defects with early and average redistribution, the segments contralateral to those with defects showing late redistribution more often had a critical stenosis supplying that segment. The frequency of myocardial infarction on ECG and the number of segments with akinetic and dyskinetic wall motion were less in defects undergoing late rather than no redistribution. Thus the time to completed TI-201 redistribution following stress appears to be related to the severity of stenosis in the coronary artery supplying the defect. Also, late redistribution is associated with the presence and early redistribution with the absence of a significant stenosis in the coronary artery to the contralateral segment.