Alemtuzumab is highly effective in relapsing remitting multiple sclerosis (RRMS), but autoimmune adverse events are of concern. In contrast to rare cases of immune-mediated cutaneous vasculitis, systemic vasculitis after alemtuzumab has not yet been described. We report the case of a 29-year-old man with RRMS who developed fever, auricular chondritis, cutaneous vasculitis and life-threatening diffuse alveolar haemorrhage, 12 months after alemtuzumab. Antibodies to myeloperoxidase appeared 9 months after alemtuzumab and were extremely high at the time of vasculitis. Outcome was favourable after glucocorticoids, plasma exchanges and rituximab. Thus, alemtuzumab may induce life-threatening vasculitis in patients treated for RRMS.