Campath 1-H (Alemtuzumab) is a humanised monoclonal antibody which targets the CD52 antigen, a low molecular weight glycoprotein present on the surface of most lymphocyte lineages, causing complement mediated lysis and rapid and prolonged T lymphocyte depletion. Following encouraging initial data from other centres we report our open label experience of using Campath 1-H as a treatment in aggressive relapsing multiple sclerosis in a consecutive series of 39 highly selected patients treated across three regional centres and followed for a mean of 1.89 years. The mean annualised relapse rate fell from 2.48 pre treatment to 0.19 post treatment with 29% of documented relapses observed in the 12 weeks following initial infusion. Mean change in EDSS was -0.36 overall and -0.15 in those patients completing > or =1 year of follow- up. Eighty-three per cent of patients had stable or improved disability following treatment. Infusion related side effects were common including rash, headache and pyrexia but were usually mild and self limiting. Transient worsening of pre-existing neurological deficits during infusion was observed in 3 patients. 12 patients developed biochemical evidence of autoimmune dysfunction, 2 patients developed thyroid disease and 1 patient autoimmune skin disease. We conclude that relapse rates fall following Campath 1-H. Whilst side effects were common these were normally self limiting or easily managed, suggesting Campath 1-H may be of use in the treatment of very active relapsing remitting multiple sclerosis.