Thallium-201 reinjection following 3-4-hr redistribution imaging improves the detection of viable myocardium but considerably prolongs the total investigation time. We compared the results of immediate 201Tl reinjection with 3-hr redistribution imaging in 120 consecutive patients who were evaluated for myocardial ischemia. Thallium-201 was reinjected immediately following the postexercise study. The images were acquired 1 hr later and reacquired at 3 hr redistribution. A total of 960 segments per imaging series were evaluated, of which 320 (33%) segments showed perfusion defects on the post-exercise images. On the 1-hr images, 220 (69%) segments demonstrated enhanced thallium uptake, 97 (30%) segments did not change and 3 (1%) segments showed reverse redistribution. Of the 100 (97 + 3) persistent defects, only 12 (4%) segments showed fill-in of 201Tl on 3 hr redistribution images. A total of 49 (15%) segments showed reverse redistribution. Defects on postexercise images were seen in 95 patients (79%) of whom 9 (10%) showed no change on immediate reinjection images. In only 1 (1%) patient was the diagnosis changed from myocardial necrosis to myocardial ischemia after analysis of the 3-hr redistribution images. These data indicate that immediate postexercise reinjection of 201Tl followed by 1-hr image acquisition may be superior to 3-hr redistribution imaging in identifying viable myocardium in patients evaluated for myocardial ischemia. This protocol eliminates the need for an additional series of 3-4-hr redistribution images and offers the advantages of reduced total imaging time, improved convenience for the patient and increased patient throughput.