Routine clinical and laboratory data were collected from 70 patients with first myocardial infarcts. Compared with the 35 patients whose chest pain duration was less than eight hours, those with chest pain eight hours or longer had higher rises in creatine phosphokinase and SGOT levels, greater summed ST segments (taken from the 12-lead ECG), more transmural infarcts, greater cardiac volumes (measured from subsequent chest roentgenograms), higher temperature rises, and higher WBC counts. These reuslts suggest that pain duration may be an indicator of infarct size.