Background: Longitudinally extensive transverse myelitis (LETM) is a demyelinating condition that is associated with diseases such as neuromyelitis optica spectrum disorder (NMOSD), acute disseminated encephalomyelitis, collagen vascular disease, or can be idiopathic. LETM can be severe enough to cause quadraparesis, marked sensory dysfunction, and respiratory failure. Rarely, these patients are unresponsive to conventional immune therapy. Methods: We report two cases of severe LETM with acute development of quadraparesis and respiratory failure requiring intensive care admission and failure to respond to high-dose corticosteroids, plasma exchange, IVIg and rituximab. Disease cessation and ultimately, significant recovery, was achieved after an 8-day cyclophosphamide induction. Results: A 21 yo female with antibody positive NMOSD and a 19 yo male with idiopathic LETM remained quadraparetic and ventilator dependent with active MRIs despite multiple courses of intravenous methylprednisolone, plasma exchanges, and in the NMOSD patient, IVIg and a 4-week course of rituximab. Both patients ultimately improved significantly and are now ambulatory with subsequent cyclophosphamide induction. Conclusions: In patients with severe LETM of presumed immune origin, who fail to respond to corticosteroids and plasma exchange, cyclophosphamide induction should be considered. This agent provides a more robust immunosuppressive response and can be induced rapidly. Cyclophosphamide effects and supportive evidence are further discussed.