In order to evaluate myocardial viability, we developed a new method for quantification of redistributed area in Tl-201 myocardial SPECT, and it was applied to 71 patients with LAD lesion. Initial image was subtracted from delayed image, and redistributed area was displayed as an unfolded map (viability map). Extent and viability score, which correspond to extent and degree of the viable area, was also calculated. In the cases with prior myocardial infarction, viability score was widely distributed and extent score was smaller, while the cases with angina pectoris had larger extent score. Visual assessment of redistribution to the infarcted area was compared with % score, which was calculated as ratio to initial defect score. When the criteria of viability was defined as % viability score greater than 36 or % extent score greater than 16, quantitative evaluation agreed with visual assessment, and corresponded to clinical course. This method was considered to be effective for evaluation of extent and degree of myocardial viability, but further evaluation need to be done in comparison with reinjection, PET study, and so on, and usefulness in multi-vessel disease.