We report a case of severe acute cellular rejection of the cardiac allograft conduction system in a 15-month-old girl who received orthotopic heart transplantation (OHT) for congestive heart failure from a congenital heart block. Post-operatively, the patient was treated for clinical evidence of rejection, but did not have electrocardiographic findings of heart block. Six weeks after transplantation, the patient developed sudden-onset bradycardia and died. Autopsy showed severe acute cellular rejection involving primarily the conduction system. Cellular rejection of the cardiac conduction system is a potentially lethal complication of OHT. Although diagnostic modalities to predict or detect ongoing cellular rejection in the conduction system are limited, recognizing the early signs, such as post-operative heart block, may prevent devastating consequences.