A new method for non-invasive monitoring of acute rejection after heart transplantation is presented. The method is basead on the evaluation of telemetrically obtained epimyocardial electrograms using implanted pacemakers and electrodes with fractal surface structure. Parameters derived from the ventricular evoked response from stimulated heart beats are significantly related to acute cellular rejection of grade 2 and higher, as confirmed by endomyocardial biopsy. The results with regard to sensitity (93 por cento) and specificity (76 por cento) emphasize the potential of this new method for substantially reducing the number of endomyocardial biopsies, that are up to now required for a safe rejection surveillance in transplant patients.