Tumor lysis syndrome (TLS) defines the metabolic derangements that occur with tumor breakdown following the initiation of cytotoxic therapy. TLS results from the rapid destruction of malignant cells and the abrupt release of intracellular materials and their metabolites into the extracellular space. The syndrome causes hyperuricemia, hyperkalemia, hyperphosphatemia, secondary hypocalcemia and uremia. It can result in acute renal failure and be fatal. Early recognition of patient at risk and preventive measures are important. There is a high incidence of TLS in tumors with high proliferative rates and large burden such as acute lymphoblastic leukemia and Burkitt's lymphoma. It less commonly occurs in solid tumors such as testicular cancer, breast cancer and small cell lung cancer. There are only a few reports on TLS complicated in CML in blast crisis. So we report a 45-yr-old woman presenting with TLS associated with CML in lymphoblastic crisis after the initiation of cytotoxic chemotherapy.