Electrocardiographic and echocardiographic findings were compared in 44 patients with a first transmural infarction. Each patient was investigated on days 1, 2, 10, and 360. The electrocardiogram was classified according to QRS and ST segment changes. Local left ventricular function was determined from mean systolic wall velocity measurements by an M mode echocardiographic mapping technique in 10 of 16 segments suitable also for electrocardiographic evaluation. Mean systolic wall velocity was corrected for differences in anterior and inferior wall motion. Wall motion was normal in segments without QRS or ST changes throughout the study. All segments with QRS or ST changes showed significantly lower corrected systolic wall velocity values during the acute stage. Segments with ST depression, alone or in combination with a minor Q wave, had corrected mean systolic wall velocity values similar to those of normal segments after one year. Segments with major Q waves and all segments with ST elevation showed reduced corrected mean systolic wall velocity values throughout the study. Segments with ST elevation, irrespective of Q waves, showed the most severely reduced wall motion with significantly lower corrected mean values than segments with minor or major Q waves without ST elevation on days 10 and 360. Thus when electrocardiograms are used for defining local left ventricular function, consideration must be given to the phase of illness, QRS morphology, and presence of ST segment elevation.