Context: Long extensive transverse myelitis (LETM) seldom develops in patients with breast cancer who are aquaporin-4 antibody (Aqp-4 Ab)-positive. Whether this association is coincidental is not well understood. Findings: A 62-year-old woman presented with treatment-resistant LETM and Aqp-4 Ab. Two months later, a stage 3 invasive ductal carcinoma was detected in her right breast. Following tumor resection and chemotherapy, her neurologic symptoms and magnetic resonance imaging findings significantly improved and serum Aqp-4 Ab disappeared. The breast tumor samples of this patient and neurologically normal patients showed inflammatory infiltrates and Aqp-4 expressing cells. Conclusion/Clinical Relevance: The temporal association between tumor treatment, amelioration of clinical findings, and seroreversion suggest that coexistence of cancer and LETM is not coincidental. Cancer screening should be considered at least in treatment-resistant LETM cases.