To evaluate diagnostic accuracy of Tc-99m-pyrophosphate (PYP) myocardial scan, we analysed 160 Tc-99m-PYP scans (acute transmural myocardial infarction 87 cases, acute subendocardial infarction; 20 cases, unstable angina pectoris; 7 cases, other disease; 46 cases). These scans were requested by the physician in Seoul National University Hospital from Sep. 1982 to Oct. 1987. And the diagnosis was confirmed by clinical course and laboratory examinations. 1) The diagnostic sensitivity of Tc-99m-PYP scan in acute transmural myocardial infarction was 91.2% (62/68) if scintigraphy was performed within 7 days after infarction, 57.1% (8/14) between 8th and 14th day, 20% (1/5) and after 15 days. 2) The diagnostic sensitivity of Tc-99m-PYP scan in acute subendocardial infarction was 75% (12/16) if scintigraphy was performed within 7 days after infarction and 0% after 8 days. 3) The diagnostic specificity of Tc-99m-PYP scan in acute myocardial infarction was 94.3% (5/53). Among 5 cases of false positive scans, 1 case was unstable angina pectoris, 2 cases were old myocardial infarction with left ventricular aneurysm, 1 case was old myocardial infarction and the remaining 1 case was cardiomyopathy.