Apparent washout of T1-201 may occur between redistribution and reinjection images. To examine the frequency of it, we prospectively compared 4-hour redistribution and reinjection images in 63 consequent patients. Al1 patients underwent pharmacological stress test using 0.56 mg/kg dipyridamole. Immediately after the 4-hour redistribution images, 1mCi T1-201 was injected at rest, and images were reacquired 10 minutes after reinjection. The stress, redistribution, and reinjection images were then analyzed semiquantitatively (0=no uptake, l=faint uptake, 2=mildly diminished uptake, 3=normal uptake). Of the 100 abnormal myocardial regions on the stress images, 54 showed either complete or partial reversibility on 4-hour redistribution images. After reinjection 11(21, 2%) of these regions demonstrated apparent T1-201 washout due to low differential uptake of the tracer, Such lesions would appear irreversible if redistribution imaging is not performed before reinjection. Thus 4-hour redistribution imaging should be performed for assessment of myocardial ischemia and viability.