To evaluate whether changes in QRS voltage reflect changes in left ventricular function after myocardial infarction, 28 patients were studied following anterior myocardial infarction. Two-dimensional echocardiograms and 12-lead electrocardiograms were obtained during the acute phase of the infarction and again after at least 30 days of recovery (mean, 8 ± 8 months). At follow-up, 11 patients (group A) showed improvement in left ventricular systolic function; 9 had increased net QRS voltage in V1-6 and 8 in V1-4. No improvement in ventricular function was found in 17 patients (group B); 7 had increased QRS voltage in V1-6 (p < 0.05 vs group A) and only 5 in V1-4 (p < 0.05 vs group A). For detection of improved left ventricular function, the sensitivity, specificity, and predictive value of the change in net QRS voltage for leads V1-6 was 82%, 59%, and 56% respectively, and for leads V1-4 was 73%, 71%, and 62% respectively. Neither R wave voltage, Q wave voltage, nor the total number of Q waves was reliable for identifying patients with improving left ventricular function. Thus, increasing net QRS voltage in the precordial electrocardiographic leads during long-term follow-up after anterior myocardial infarction correlates with and has a reasonable sensitivity for detection of improvement in left ventricular systolic performance.