Although immunosuppressive effects of antiepileptic drug are well known, serious complications of antiepileptic drug are rare. We report a case of hypogammaglobulinemia associated with aplasia of B lymphocytes after carbamazepine treatment. Despite repeated intravenous immunoglobulin replacement therapy, this condition persisted for more than three months. The present case suggested that routine monitoring of the blood cell count and serum levels of immunoglobulin are important in patients treated with carbamazepine, and a lymphocyte subpopulation study is valuable in cases of hypogammaglobulinemia.