Reverse redistribution(RRD) refers to a perfusion defect that develops or becomes more evident on rest imaging compared with the stress imaging. This phenomenon was not uncommonly noted on myocardial perfusion single photon emission computed tomography(SPECT). However, the clinical significance and pathophysiological mechanism of RRD were unclear. The aim of this study was to evaluate the incidence and clinical significance of RRD on either dipyridamole T1- 201 or Tc-99m MIBI myocardial perfusion SPECT. RRD was defined as≥10% decrease in relative T1-201 and Tc-99m MIBI uptakes on rest images compared to the stress images or as an appearance of new perfusion defects on rest images. It was observed in both T1-201 (44/463, 9.5%) and Tc-99m MIBI (124/999, 12.4%) myocardial SPECTs similarly, with an overall incidence of 11.5% (168/1462). Many apparently unrelated disease groups showed the finding: post-revascularization(53.9%), coronary artery disease(24.6%), myocardial infarction(l2.3%), and those with normal coronary arteries(9.2%). Clinical and angiographic characteristics of 65 consecutive patients who underwent coronary arteriography in 168 patients who had RRD on myocardial perfusion SPECT were reviewed. Tc-99m MIBI was used in 44 patients, and TI-201 was used in 21 patients. Of the 81 myocardial segments analyzed which showed RRD, 32 segments(39.5%) were in septum, 24(29.5%) in inferior wall, 12(14.8%) in anterior wall, 7(8.7%) in apex and 6(7.4%) in lateral wall. There was no clear association between RRD and coronary arterial stenosis or presence of collateral circulations. Ventriculographical wall motion was evaluated in 27 regions with RRD; it was normal in 12 regions, hypokinetic in 12 regions and dyskinetic in 3 regions. In 14 of 21 patients who showed RRD on T1-201 myocardial SPECT, T1-201 reinjection was performed immediately after the 3-4 hour redistribution studies. Ten of 14(71.4%) showed enhanced T1-201 activity(≥10% increased) after reinjection. We conclude that RPD is not related to mode of stre