We performed a retrospective analysis of 42 consecutive patients undergoing autologous BMT to determine the incidence of second and third degree heart block following the infusion of cryopreserved autologous bone marrow and to identify any predisposing characteristics. A decrease in heart rate > or = 10 beats/min was observed in 80.5% of patients, with a mean decrement of 27 +/- 7 beats/min. 48.8% of patients developed absolute bradycardia (< or = 60 beats/min). Four of 41 patients (9.7%) experienced high-grade heart block: 9.7% second degree and 4.8% third degree. Heart block patients did not differ from the non-heart block group with respect to age, interval from diagnosis or bone marrow harvest to transplant, cardiac risk factors, pretransplant electrocardiograms or radionuclide angiograms, transplant chemotherapy regimens or serum chemistry values. There was an increased incidence of heart block in patients with prior exposure to cyclophosphamide (p < 0.05) and vinca alkaloids (p < 0.05). There appears to be a high incidence of transient second and third degree heart block following autologous marrow infusion. This may be related to prior chemotherapy, but more likely is an effect of the infusate itself. Predisposing factors were not identified.