Objective: To examine the reproducibility, construct validity, and unidimensionality of the Dutch translation of the de Morton Mobility Index (DEMMI), a performance-based measure of mobility for older patients. Design: Cross-sectional study. Setting: Rehabilitation center (reproducibility study) and hospital (validity study). Participants: Patients (N=28; age >65y) after orthopedic surgery (reproducibility study) and patients (N=219; age >65y) waiting for total hip or total knee arthroplasty (validity study). Intervention: Not applicable. Main Outcome Measures: Not applicable. Results: The intraclass correlation coefficient for interrater reliability was high (.85; 95% confidence interval, 71.93), and minimal detectable change with 90% confidence was 7 on the 100-point DEMMI scale. Rasch analysis identified that the Dutch translation of the DEMMI is a unidimensional measure of mobility in this population. DEMMI scores showed high correlations with scores on other performance-based measures of mobility (Timed Up and Go test, Spearman r=-.73; Chair Rise Time, r=-.69; walking test, r=.74). A lower correlation of.44 was identified with the self-report measure Western Ontario and McMaster Universities Osteoarthritis Index. Conclusions: The Dutch translation of the DEMMI is a reproducible and valid performance-based measure for assessing mobility in older patients with knee or hip osteoarthritis. © 2011 American Congress of Rehabilitation Medicine.