A 51-year-old man who underwent umbilical cord blood transplantation for acute lymphoblastic leukemia with a Philadelphia chromosome in April 2006 achieved complete remission. In June 2008, progressive renal dysfunction and melena emerged and the patient was diagnosed with B-cell-type malignant lymphoma. He presented with severe hypo-phosphate- mia(0. 1m g/dL)due to paraneoplastic syndrome, simultaneously. Because the development of tumor lysis syndrome followed by hyper-phophatemia was feared to occur after CHOP chemotherapy, we discontinued the adjustment of serum phosphorus. The serum phosphorus level was elevated to 11.6mg/dL after 3 days and decreased to 3. 8mg/dL after 5 days. We must be careful regarding hyper-phosphatemia and phosphorus adjustment even in patients with severe hypo-phosphatemia that is due to paraneoplastic syndrome.