QUESTION: Is the de Morton Mobility Index (DEMMI) valid for measuring the mobility of patients living in the community with Parkinson disease (PD)? DESIGN: A prospective cohort study comparing the DEMMI with existing mobility-related activity measures. PARTICIPANTS: 102 consecutive patients with PD (mean age 72.4 years, SD 8.3) who received multidisciplinary outpatient care in a community rehabilitation facility over 6 to 8 weeks. OUTCOME MEASURES AND ANALYSIS: The DEMMI and other measures of mobility-related activity at admission and discharge (10 metre walk test, Berg balance scale, timed up and go test, functional reach test and pull test) were used to assess convergent validity. Discriminant validity of the DEMMI was assessed by comparison to measures of other constructs, and. unidimensionality was examined using Rasch analysis. RESULTS: Evidence of convergent (rho ranged from 0.47 to 0.84), discriminant (rho ranged 0.12 to 0.22), and known groups validity were obtained for the DEMMI (Mean difference 20.3 units, 95% CI 14.5 to 26.0). Rasch analysis confirmed that the DEMMI was unidimensional. CONCLUSION: The DEMMI has a broader scale width than existing mobility-related activity measures, provides interval level measurement and is a unidimensional measure of mobility in patients with PD living in the community.