The prevalence of arrhythmias after orthotopic cardiac transplantation in the era of cyclosporine immunosuppression is unknown. Accordingly, we analyzed telemetry data from the initial hospitalization period for 33 cardiac transplant procedures and from 100 hospital readmissions in 23 long-term survivors. Prevalence of arrhythmias was analyzed in relation to immunosuppressive regimen, occurrence of acute rejection, and other clinical and hemodynamic variables. Atrial and ventricular arrhythmias were recorded in 55% and 79% of patients during initial hospitalization and in 39% and 43% of long-term survivors, respectively. Compared with cyclosporine-treated patients, atrial (24% vs. 88%, p less than 0.001) and ventricular (65% vs. 94%, p = 0.085) arrhythmias during the initial hospitalization were more prevalent in patients receiving azathioprine. There was no difference in the prevalence of arrhythmias during initial hospitalization between patients experiencing acute rejection and those who did not. Potential associations between arrhythmia occurrence and a prolonged donor heart ischemic time (p = 0.022), elevated pulmonary arterial pressure (p = 0.01), and a lower ejection fraction (p = 0.009) were noted. These data suggest that arrhythmias occur in the majority of patients during the initial hospitalization for transplantation, even in those who never experience acute rejection. Ventricular arrhythmias are common in patients treated with either immunosuppressive regimen; however, atrial arrhythmias are less prevalent in patients receiving cyclosporine. Arrhythmias tend to occur in the setting of altered hemodynamics.