Abstract: BACKGROUND: Tumor lysis syndrome (TLS) is an oncological emergency associated with hematological malignancies or highly proliferative solid tumors, commonly after chemotherapy. It is rarely associated with transient abnormal myelopoiesis. OBSERVATION: We report a rare case of a neonate with transient abnormal myelopoiesis and tumor lysis syndrome, complicated with concomitant heart failure due to an underlying atrioventricular septal defect. Hyperhydration was contraindicated due to heart failure. The patient was managed conservatively with full recovery. CONCLUSION: Tumor lysis syndrome should be suspected in neonates with transient abnormal myelopoiesis with electrolyte abnormalities. Treatment options should be considered carefully for their risks and benefits.