One hundred and three patients with myocardial infarctions were studied with 201-thallium chloride and/or 99m-technetium pyrophosphate myocardial imaging and were followed-up for an average of 23 months. There were 24 false negative cases with 201-thallium chloride, but no deaths or serious complications occurred during the follow-up period in these false negative cases. There were six patients with widened QRS complexes (more than 0.12 seconds) without bundle branch blocks and in three of them myocardial infarction was not identified by electrocardiography. However, there were large myocardial perfusion defects in the anterior-inferior wall of the left ventricle. All of this group of patients died suddenly during the follow-up period. The incidence of complications and mortality rose sharply in patients whose myocardial perfusion defects detected by thallium-201 were larger than 40% of the entire left ventricle. The myocardial infarction areas measured by 99m-technetium pyrophosphate were 28.5 +/- 9.8 cm2 in non-survivors and 16.5 +/- 1.7 cm2 in survivors. In addition, ten patients with acute myocardial infarction were studied by double scan methods with thallium-201; myocardial perfusion defect areas were reduced from 29 +/- 3% of the entire left ventricle to 19 +/- 4% by nitrate administration, indicating that there were reversibly ischemic areas in acute myocardial infarction which could be transiently reduced by nitrate. Thus, the study suggests the possibility of decreasing myocardial perfusion defects in the early phase of acute myocardial infarction, leading to a better long term prognosis for the patients.