The value of a change in summated electro-cardiographic voltage as a predictor of cardiac rejection is uncertain in patients treated with low-dose triple immunotherapy (cyclosporin, azathioprine, and corticosteroids) following orthotopic cardiac transplantation. Ten recipients were studied daily and the summated QRS voltages from 600 electro-cardiograms were calculated. A single observer graded 147 endomyocardial biopsies. During the study period there were 18 episodes of acute rejection with myocyte necrosis. Only three episodes of rejection were heralded by a significant change in QRS voltage, yielding a positive predictive value of 13% for the technique. Three episodes of severe bacterial infection were preceded by significant decreases in QRS voltage. In one patient with a global pericardial effusion, QRS voltage was related to the depth of effusion measured by echocardiography. These data show that QRS voltage is of extremely limited value in the prediction of cardiac rejection in patients treated with low-dose triple immunotherapy. © 1989 Springer-Verlag.