Tumor lysis syndrome is a massive lysis of malignant cells due to quick proliferation, characterized by hyperuricemia, hyperkalemia, and hyperphosphatemia. Hyperphosphatemia may lead to hypocalcemia. Tumor lysis syndrome usually develops after the start of chemotherapy and may lead to acute renal failure and death, due to renal tubular precipitation of uric acid and other complications. The risk factors for development of tumor lysis syndrome are malignancies with large growth fraction (i.e., B-cell and T-cell lymphomas/leukemias), large tumor bulk, use of cytolytic chemotherapy, pre-existing renal insufficiency, kidney involvement, ureteral or bladder obstruction by the tumor, and metabolic abnormalities. Prevention consists of hydration for high urinary flow and urinary alkalinization with sodium bicarbonate to prevent acid uric precipitation in renal tubules. Allopurinol is used in older children to inhibit xanthine oxidase and decrease uric acid formation, but not in concomitance with rasburicase. Although allopurinol has been used in neonates for other serious health conditions, it has not been used or researched in neonates or in infants for prevention of tumor lysis syndrome (Schmidt CWP. Chemotherapy in neonates and infants: pharmacological oncology for children under 1 year old. Springer, Cham, 2018).