AbstractIn a consecutive series of 401 patients admitted to the Coronary Care Unit of the Copenhagen County Hospital at Glostrup suspected of acute myocardial infarction (AMI) less than 24 hours old, blood samples for enzyme analysis were drawn at admission and 12, 24, 36, 48, and 72 hours later, and ECG in 9 leads was recorded daily during the first week. From usual diagnostic criteria — clinical symptoms, ECG‐findings, and enzyme analyses, not including and not knowing the results of CK—MB analyses — the series was divided into patients with AMI and those without AMI.Comparison is made between the predictive values in the diagnosis of AMI of a positive and a negative result (PV‐pos. and PV‐neg.) of either of the tests: ECG and serum levels of aspartate aminotransferase (ASAT), lactate dehydrogenase (LD), creatine kinase (CK), and MB‐fraction of CK determined by electrophoresis. The salient features were: PV‐neg. i.e. diagnostic sensitivity was low (0.78) for ECG, but high for all enzyme tests (0.97–1.00). PV‐pos. i.e. diagnostic specificity was rather low for the three standard enzymes (0.73–0.87), but high for CK—MB (0.98) and also for ECG (1.00). Thus, determination of CK—MB seems to offer the best combination of a high sensitivity as well as specificity in the diagnosis of AMI. However, the time factor should be kept in mind, as positive results of CK—MB can only be expected during the first 30–50 hou