Myocardial washout of the new myocardial perfusion imaging agent, 99mTc-hexakis-2-methoxy isobutyl isonitrile (99mTc-MIBI) was studied in 23 patients with coronary artery disease. These patients were divided into three groups; 8 patients with effort angina pectoris (AP group), 6 with myocardial infarction who had reversible defect (MI-RD (+) group) and 9 with myocardial infarction who had not reversible defect (MI-RD (-) group). Regional radioactivities were determined with postexercise 1- and 3-hour myocardial planar images. In AP group, 99mTc-MIBI washout rate in ischemic area was smaller than in normal area (3.8 +/- 7.5% vs. 8.3 +/- 5.6%, p < 0.05), however, in the remaining two groups, there were no difference in washout between normal and infarct areas. Defect/normal ratios from postexercise 1 hour to 3 hours were slightly increased in AP and MI-RD (+) groups. Our results indicate that 99mTc-MIBI showed slight and incomplete myocardial redistribution in ischemic areas. These data showed to be considered in the interpretation of 99mTc-MIBI myocardial imaging.