A higher incidence of arrhythmias was observed in 16 renal allograft recipients infected with Cytomegalovirus (CMV), as compared to 26 patients who were transplanted during the same period and did not contract the infection (43.7% vs. 3.8%; p less than 0.01). There was a highly significant difference in the incidence of sinus tachycardia (37.5% infected vs. 0% not infected; p less than 0.005), while the incidences of atrial fibrillation and ventricular ectopic beats were not significantly different. The arrhythmias always appeared in the early phases of the infection and disappeared after a certain period, lasting from one week to months. It is proposed that arrhythmias arising during CMV infection may be due to a myocardial CMV involvement.