OBJECTIVE Enhanced washout of (99m)Tc-SESTAMIBI (MIBI) is found in the myocardium in patients after acute myocardial infarction (AMI) or in those with serious angina. However, a standard index for washout evaluation in ischemic heart disease has not been established. We approached the establishment of a standard index for regional washout in ischemic heart disease and report the evaluation results of a newly developed washout evaluation method. METHODS We made a polar map from short-axis myocardial SPECT images and developed a washout index (WO INDx) based on early and delayed images. The control group consisted of 10 healthy volunteers and a patient group of 43 patients with AMI or angina. Three nuclear cardiology specialists interpreted early and delayed images and visually graded the regional uptake of MIBI in 17 segments on a polar map, and the washout rate (WR) was compared with WO INDx. RESULTS WO INDx and WR in the control group were 1.83 ± 1.95 and 35.59 ± 6.97, respectively. In the AMI cases the correlation of ejection fraction (EF) and WO INDx was -0.602, and the correlation of EF and WR was -0.346. The agreement between observers in the visual evaluation was high with excellent to moderate agreements. The ROC analysis was performed for WS2 with a washout score of 2 in the visual evaluation by Observers 1 to 3. The area under the ROC curve (AUC) was 0.934, 0.949 and 0.934 for WO INDx, respectively, and 0.681, 0.662 and 0.656 for WR, respectively, indicating that the AUC was higher for WO INDx. The sensitivity for WO INDx was 89.3, 88.9 and 96.3%, respectively, and the specificity was 88.2, 89.8 and 79.3%, respectively. The sensitivity for WR was 53.6, 52.8 and 51.9%, respectively, and the specificity was 87.5, 79.4 and 87.4%, respectively. These results suggested that WO INDx had higher reliability than WR in terms of sensitivity. CONCLUSIONS The results suggested that the diagnosis using a new index, WO INDx, calculated from standardized percentage uptakes is more useful than that using the washout rate determined from the myocardial count in the MIBI washout evaluation.