In 38 patients with acute anterior myocardial infarction or ischaemia, the extent and amplitude of ST segment elevation was measured on the chest wall with a praecordial mapping technique. There was a poor correlation with measurements of clinical severity and with the extent of necrosis as measured by the peak levels of creatine kinase (CK) and the estimated total amount of CK released. In two patients ST segment re-elevation occurred without re-elevation of serum CK and in two other patients reinfarction obvious on enzyme re-elevation occurred without changes in ST segments. Praecordial ST segment mapping appears to have a limited role in measuring or monitoring human infarct size and would be an unreliable tool for evaluation of methods to limit myocardial necrosis. The chest leads of the standard 12-lead electrocardiogram provided sufficient information for clinical evaluation of ST segment elevation.