We compared creatine kinase MB (CK-MB) mass and total creatine kinase (CK) sampled three times daily with conventional cardiac enzymes. The influence of the electrocardiogram (ECG) on admission, frequency of blood sampling, thrombolytic therapy, different upper reference limits of the biochemical markers and duration of symptoms were assessed in 100 consecutive patients with suspected AMI of whom 63 were confirmed according to WHO criteria. Early sensitivity but not specificity of CK-MB mass, with and without ECG, for cut points <8 microg/l was significantly better than total CK sampled frequently. The sensitivity of ECG on admission (52%) was significantly improved by CK-MB analysis (79%) but not by total CK. Duration of symptoms (range of means 3.5-9 h) or thrombolytic treatment had no influence on the sensitivity and specificity of CK-MB mass. In AMI with inconclusive ECG, CK-MB mass performed best of the markers with a sensitivity of 70% versus 17% of total CK (P<0.001) on admission. CK-MB mass was also elevated in 8 patients classified conventionally as unstable angina. We conclude that CK-MB mass is a more useful marker of AMI during the first 16 h of chest pain than frequently sampled total CK, ECG and conventional cardiac enzymes.