Methodology: We propose to recruit 120 people with SPMS in whom progression rather than relapse is the major cause of increasing disability into a double blind parallel group controlled trial lasting two years in which random allocation would be made to receive treatment with either lamotrigine or placebo. We anticipate that patient recruitment, follow-up and trial management could be achieved readily across four proposed sites in London. The primary endpoint would be an effect of treatment on cerebral atrophy, which correlates with other MR markers of axonal loss, and which can be measured reliably and sensitively using recently developed MR techniques. The trial is powered to detect a 60% beneficial effect on the rate of development of cerebral atrophy. Secondary endpoints would include effects of treatment on spinal cord atrophy and on clinical measurements of impairment/disability. MR measures of brain volume and cervical spinal cord cross-sectional area and scores of clinical impairment/disability would be determined at entry, and then after 12 and 24 months. Brain volume would be measured additionally at 6 and 18 months. Clinical follow-up would occur every 3 months, and interim analysis is planned at 12 months.