Relationship between the duration of coronary artery occlusion and myocardial infarct size was investigated in 24 patients with acute anterior myocardial infarction associated with occlusion of the left anterior descending artery. The duration of coronary artery occlusion was divided into (A) 4 hours or less, (B) 4-10 hours, and (C) 10 hours or more. Defect score obtained by thallium-201 emission computed tomography was significantly greater, and left ventricular ejection fraction was significantly lower as the duration of coronary artery occlusion increased. Creatine kinase-MB (..sigma..CK-MB) was higher in cases of longer duration of occlusion. However, this was not significant between the groups A and B, suggesting the influence of reperfusion on the ..sigma..CK-MB release. The duration of coronary artery occlusion was considered to be an important factor to determine the infarct size, and significance of early reperfusion was suggested.