99mtechnetium diphosphonate was used in 81 patients for myocardial imaging. 46 of the patients suffered, 2-6 days prior to the test, from an acute transmural infarction provided by clinical, ECG, and enzymatic evidence; the scintigram was definitely positive in 43, equivocal in 2, and negative in 1. In 16 patients with subendocardial infarctions, in 9 the scintigram showed a positive result, in 1 it was equivocal, and in6 patients negative. In 6 other patients who suffered from acute chest pain, followed by enzyme changes, but no ECG evidence of infarction throughout the hospitalization period, the scan was positive in all. In 13 patients with acute ischemia (unstable 'crescendo' angina), 9 had a positive, 1 an equivocal, and 3 a negative scan. We conclude that radionuclide imaging is a valuable addition to the methods already available for diagnosis of myocardial infarction. With the use of diphosphonate as the tracer, we found that acutely ischemic myocardium that later may recover, gives also positive imaging.