The incidence and prevalence of tremor in multiple sclerosis is difficult to estimate accurately, although tremor of moderate and severe magnitudes were found in 32% and 6% respectively of patients in one study.22 In part this is because of the problem of distinguishing intention tremor from serial dysmetria, which is the result of the voluntary sequential correction of movement errors, and some types of postural tremor from other postural instabilities.27 In addition, the natural history of multiple sclerosis and in particular the transience of the neurological signs during the relapsing and remitting phase make prevalence studies difficult. This problem is compounded by the structure of the Kurtzke functional systems deployed for the assessment of patients with multiple sclerosis, because subscale part B (cerebellar function) does not isolate tremor.28 In a 3 year follow up study of multiple sclerosis, cerebellar deficits of functional importance were found to occur in 33% of 259 patients and to be predictive of a worse prognosis.29 30 A similar proportion was found to have ataxic symptoms in an extensive epidemiological survey undertaken in the United Kingdom and involving over 300 patients with multiple sclerosis.26 There are no published quantitative studies of the contribution of tremor to the disabilities and handicaps caused by the disease.