A 38-year-old man was admitted to our hospital with neck pain, dysesthesia of both hands, and weakness of the left upper limb. He had been diagnosed with a chronic active Epstein-Barr virus infection (CAEBV) at the age of 34 and had undergone umbilical cord blood transplantation at the age of 37. MRI of the spinal cord revealed an intramedullary hyperintense lesion on T 2 -weighted images with gadolinium enhancement. Because his laboratory tests revealed proliferation of CD19 + lymphocytes in

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... e peripheral blood, and EBV DNA was detected in both peripheral blood and CSF, he was diagnosed as having post-transplant EBV associated lymphoproliferative disease. However, chemotherapy did not alleviate his symptoms. At a later time, quantitative chimerism analysis of his CSF showed a higher proportion of lymphocytes that had originated from the recipient. Finally, he was diagnosed as having a recurrence of CAEBV in the central nervous system, and his symptoms were restored by intrathecal chemotherapy (methotrexate, cytosine arabinoside, and prednisolone). Quantitative chimerism analysis of CSF was useful for diagnosing the recurrence of CAEBV in the central nervous system. (Clin Neurol 2014;54:809-813)