Alemtuzumab is an anti-CD52 monoclonal antibody recently licensed for use in relapsing–remitting multiple sclerosis. Here, we report our experience of its use in neuromyelitis optica (NMO) spectrum disorders. A retrospective case review of patients treated with alemtuzumab in Cambridge, UK, was conducted to identify those who fulfil the criteria for NMO spectrum disorder. Three cases were identified. Case 1, 9-year-old female, presented with transverse myelitis and bilateral optic neuritis, with one lower medullary and several longitudinally extensive cord lesions. Despite immunosuppression including two courses of alemtuzumab, she continued to relapse, was wheelchair bound and registered blind by age 12, and died at age 18. Case 2, 41-year-old female, presented with bilateral optic neuritis and transverse myelitis with longitudinally extensive cervical cord lesions. Despite three courses of alemtuzumab, she had five relapses with visual impairment and new cord lesions. She later developed tumefactive white matter lesions and died aged 51. Case 3, 31-year-old female, presented with transverse myelitis with longitudinally extensive cervical cord lesions and positive aquaporin-4 antibody. After one course of alemtuzumab, she relapsed with 4 episodes of myelitis with new enhancing lesions and accumulating disability. She became relapse free after rituximab and mycophenolate mofetil. From this case series, we conclude that alemtuzumab failed to prevent disabling relapses and poor outcome in NMO. We hypothesise that rituximab is more effective, as in case 3, because it causes much more prolonged B lymphocyte depletion than alemtuzumab. We therefore caution against the use of alemtuzumab in NMO.