The application of dual tracer transaxial emission computed tomography of the heart was studied with use of technetium-99m pyrophosphate and technetium-99m-labeled red blood cells for measuring infarct size in 20 patients with acute myocardial infarction and 10 without infarction. Imaging was performed with a standard gamma camera and with a multidetector transaxial emission computed tomographic body scanner 3 hours after injection of technetium-99m pyrophosphate. Immediately after the scanning procedure, technetium-99m pertechnetate was injected to label red blood cells, and the scanning protocol was repeated. Technetium-99m pyrophosphate was detected in the anterior wall with involvement of the interventricular septum or lateral wall in patients with electrocardiographic criteria for anterior infarction, whereas uptake was detected in the diaphragmatic left ventricular wall with involvement of the posterior, posteroseptal or posterolateral left ventricle or of the right ventricle in patients with electrocardiographic criteria for inferior or posterior infarction. Infarct size measured from transaxial images ranged from 14.0 to 117.0 g in weight. There was a direct relation between infarct size and patient prognosis in that, of the 13 patients with infarct greater than 40 g, 11 (85 percent) had complications, whereas only 2 (29 percent) of 7 patients with an infarct less than 40 g had complications during a follow-up period averaging 17.8 months (p less than 0.05).