Background: Tumor lysis syndrome (TLS) is a potentially lethal oncological emergency which usually occurs in a hematological malignancy after initiation of chemotherapy. Here we present a rare case of fatal spontaneous TLS with reactive leukocytosis in a patient with localized sarcomatoid carcinoma. Case Report: A 73 years old lady a treated case of carcinoma right breast 22 years back presented with a large left infraclavicular mass. Core needle biopsy showed sarcomatoid carcinoma. Laboratory values showed elevated blood glucose, ketones, C reactive protein and procalcitonin, with features of TLS like elevated creatinine, hyperuricemia, hyperphosphatemia. She also had hypercalcemia and leucocytosis. Bone marrow studies revealed hypercellular marrow with myeloid hyperplasia. She was managed for tumor lysis syndrome and probable sepsis. Leukocytosis persisted inspite of hydroxyurea and steroids. Patient had a rapid deterioration and expired inspite of improvement in biochemical parameters. Conclusion: We present a rare case of fatal spontaneous tumor lysis syndrome with associated hypercalcemia and reactive hyperleukocytosis in a case of localized sarcomatoid carcinoma. Spontaneous tumor lysis is an extremely rare emergency in a localized solid tumour. A high suspicion in such histology and early recognition may improve patient outcome.