Tumor lysis syndrome in an onco-metabolic emergency resulting from massive lysis of rapidly proliferating malignant cells seen commonly in patients with hematological malignancies such as acute lymphocytic leukemia and Burkitt's lymphoma and is quite rare in solid tumors. Spontaneous development of tumor lysis has been described among other trigger factors such as corticosteroid therapy, anesthesia, tumor manipulation during surgery and pyrexia. We describe such a case in a 5-year-old boy posted for excision and staging of a massive Wilms' tumor who developed a hyperkalemic cardiac arrest during the procedure and its subsequent intraoperative and postoperative management. Intraoperative cardiac arrest is a stressful situation for both the anesthesiologist and the surgeon, more so when it involves a child. The aim of this report is to make the anesthesiologist aware of the possibility and occurrence of such a phenomenon in children and be adequately prepared for such an emergency.