We evaluated dual imaging with thalium-201 (201TI) and technetium-99m (99mTc) pyrophosphate in 80 patients with documented acute myocardial infarction (55 transmural, 25 nontransmural infarction). Color-coded isocount display of 201TI images was essential for interpretation in 16 patients. Combined 201 TI and 99mTc-pyrophosphate imaging for infarct detection was 100% sensitive; however, either was falsely negative in 12 of 80 patients. False-negative individual 201TI or 99mTc-pyrophosphate infarct images were most common in patients with small infacts or left ventricular hypertrophy. Thallium-201 images correctly localized the site of acute transmural infarction in all 51 patients with a positive image, while 99mTc-pyrophosphate localized the site of infarction in 49 of 53 with an abnormal image. Comparison of the size of the imaged infarct region revealed size discordance in 25 of 49 patinets, with 99mTc-pyrophosphate larger in 21 of 49 and 201TI larger in only four of 49. Thus dual radionuclide imaging provides definition of the presence and location of acute myocardial infarction.