High-frequency electrocardiograms recorded from two groups of patients who underwent exercise stress testing were examined: Group I (13 patients) and Group II (17 patients) with and without ischemic heart disease, respectively. A surface ECG was recorded in two different stages of the exercise test: during the control stage (before exercise) and during the recovery phase. The digitized waveforms from each stage were aligned using a cross-correlation schema, averaged and bandpass filtered between 150 and 250 Hz. The difference in morphology between the high-frequency QRS complexes before exercise and during the recovery phase was examined using cross-correlation analysis. The difference in morphology was significantly lower (greater c. c. coefficient) in patients without ischemic heart disease (c. c. equals 0. 826 plus or minus 0. 068) than in patients with ischemic heart disease (c. c. equals 0. 659 plus or minus 0. 114, p less than 0. 00005). It is concluded that exercise-induced changes in the morphology of the QRS complex can be detected by high-frequency ECGs and may represent a sensitive early indicator of coronary artery disease.