This study describes the refinement and validation of the 17-item DEMMI in an independent sample of older acute medical patients. Instrument refinement was based on Rasch analysis and input from clinicians and researchers. The refined DEMMI was tested on 106 older general medical patients and a total of 312 mobility assessments were conducted. Based on the results of this study a further 2 items were removed and the 15 item DEMMI was adopted. The Rasch measurement properties of the DEMMI were consistent with estimates obtained from the instrument development sample. No differential item functioning was identified and an interval level scoring system was established. The DEMMI is the first mobility instrument for older people to be developed, refined and validated using the Rasch model. This study confirms that the DEMMI provides clinicians and researchers with a unidimensional instrument for measuring and monitoring changes in mobility of hospitalised older acute medical patients.