A 63-year-old woman was admitted to our hospital with relapsed acute myeloid leukemia. On day10 after reinduction therapy, she became febrile. Computed tomography on day15 revealed right upper lobe consolidation. Because the β-D glucan and Aspergillus galactomannan antigen tests were negative, we considered pulmonary mucormycosis as a breakthrough infection under voriconazole administration. Liposomal amphotericin B was initiated, and the patient underwent unrelated bone marrow transplantation although not in complete remission. She developed right shoulder pain on day1, and her pneumonia worsened on day3. She reported right lower extremity paralysis on day15, and developed bilateral lower extremity motor and sensory paralysis the next day. T2-enhanced magnetic resonance imaging revealed hyperdense lesions in the spinal cord at Th11. Transverse myelitis was diagnosed, and she underwent antiviral therapy. After engraftment, she died of pneumonia on day24. Postmortem examination revealed disseminated mucormycosis involving the lungs, liver, diaphragm, blood vessels, and dura matter of the spinal cord; it also revealed that the sudden bilateral lower extremity paralysis was caused by disseminated mucormycosis. This case stresses the possibility of mucormycosis, particularly in prolonged neutropenic patients with pain, fever, and focal neurological findings.