The incidence and time course of complex ventricular arrhythmias and of the abnormal signal averaged electrocardiogram were studied prospectively in 90 patients in the first two months after acute myocardial infarction. Serial recordings of both 24 hour ambulatory and signal averaged electrocardiograms were obtained 0-5 days (phase 1), 6-30 days (phase 2), and 31-60 days (phase 3) after infarction. A total of 264 ambulatory electrocardiograms and 264 signal averaged electrocardiograms were available for analysis. Complex ventricular arrhythmias were seen in 31%, 17%, and 38% of patients during phases 1, 2, and 3 respectively, and abnormal signal averaged electrocardiogram in 13%, 24%, and 16%. The incidence of complex ventricular arrhythmias was not significantly different in patients with or without an abnormal signal averaged electrocardiogram in the entire study period nor in any of the three phases after infarction. During phase 2 when abnormal signal averaged electrocardiograms were most common complex ventricular arrhythmias were least common. This lack of correlation suggests that the abnormal signal averaged electrocardiogram and complex ventricular arrhythmias after infarction have different electrophysiological bases.