Objectives: (1)To describe the prevalence and severity of tremor in patients with multiple sclerosis (MS) registered within a large North American MS registry; (2) to provide detailed descriptions on the characteristics and severity of tremor in a subset of registrants and (3) to compare several measures of tremor severity for strength of agreement. Setting: The North American Research Committee on MS (NARCOMS) registry. Participants: Registrants of NARCOMS reporting mild or greater tremor severity. Outcome measures: We determined the crosssectional prevalence of tremor in the NARCOMS registry over three semiannual updates between fall 2010 and fall 2011. A subset of registrants (n=552) completed a supplemental survey providing detailed descriptions of their tremor. Outcomes included descriptive characteristics of their tremors and correlations between outcome measures to determine the strength of agreement in assessing tremor severity. Results: The estimated prevalence of tremor in NARCOMS ranged from 45% to 46.8%, with severe tremor affecting 5.5-5.9% of respondents. In the subset completing the supplemental survey, mild tremor severity was associated with younger age of MS diagnosis and tremor onset than those with moderate or severe tremor. However, tremor severity did not differ by duration of disease or tremor. Respondents provided descriptions of tremor symptoms on the Clinical Ataxia Rating Scale, which had a moderate to good (ρ=0.595) correlation with the Tremor Related Activities of Daily Living (TRADL) scale. Objectively scored Archimedes' spirals had a weaker (ρ =0.358) correlation with the TRADL. Rates of unemployment, disability and symptomatic medication use increased with tremor severity, but were high even among those with mild tremor. Conclusions: Tremor is common among NARCOMS registrants and severely disabling for some. Both ADLbased and symptom-descriptive measures of tremor severity can be used to stratify patients.