Abstract: Undiagnosed neoplasms in childhood are rare causes of sudden and unexpected death. Deaths due to undiagnosed hematologic malignancies are limited to a small number of case reports. The following case of acute leukemia was diagnosed at forensic autopsy in a 5-year-old boy with no significant past medical history. He complained of nausea and vomiting 2 days before his death, with the subsequent development of fever. Meningitis was the initial suspected cause of death. Findings at autopsy included a 100% cellular bone marrow with greater than 95% blasts. Hemorrhages involving the cerebrum, pons, epicardium, lungs, and thymus were present. Prominent leukemic infiltrates and leukostasis were present in the brain, heart, lungs, spleen, hilar lymph nodes, liver, and kidneys. A peripheral blood smear and automated blood cell count showed a white blood cell count of 435 × 109/L with greater than 80% circulating blasts. Immunohistochemical stains confirmed the diagnosis of T lymphoblastic leukemia/lymphoma. Given these circumstances, the diagnosis of acute leukemia should be considered when an intracerebral hemorrhage and/or visceral hemorrhages are identified on internal examination for appropriate collection of tissue for smears and microscopic examination. This case also highlights the uncommon, although serious, risks associated with acute lymphoblastic leukemia and hyperleukocytosis.