To evaluate the stability of precordial ST-segment mapping techniques in assessing ischemic myocardial injury, we studied 28 patients with acute anterior myocardial infarction using a 49-lead electrocardiographic system (1 mV. = 10 mm.). The sum of ST elevations in millimeters was taken as an index of ischemic injury, and remained stable in two consecutive maps made approximately one hour apart (65.8 +/- 8.4 vs. 63.8 +/- 8.7 mm.). The number of sites showing ST elevation larger than or equal to one mm. was taken as an index of extent of injury, and showed a small but statistically significant decline (23.9 +/- 4.5 vs. 22.3 +/- 4.2 mm.) during the same time interval. Blood pressure and heart rate remained stable. Changes in the map were observed in five patients, but could be explained in four by abrupt alterations in clinical status. In one patient no explanation for alterations in the map was apparent. We conclude that precordial ST segment maps are usually stable in the early stages of anterior myocardial infarction, but should be used to evaluate interventions only with careful clinical monitoring, and with the knowledge that occasional unexplained variations do occur.