We report a case of decreased ciclosporin (CsA) trough level due to the interaction with phenobarbital after living renal transplantation in a 15-year-old male. Two other cases prescribed valproate sodium as an anticonvulsant, showed no change in the plasma CsA levels. Therefore, it is necessary to recognize the drug interaction with CsA, and appropriate drug should be chosen in clinical renal transplantation.