Purpose: Many patients suffer long term loss of mobility after hip fracture but there is no gold standard method for measuring mobility in this group. We aimed to validate a new mobility outcome measure, the de Morton Mobility Index (DEMMI) in a hip fracture population during inpatient rehabilitation. Method: The DEMMI was compared with the existing measures of activity limitation: 6 minute walk test, 6 metre walk test and Barthel Index on 109 consecutive patients admitted to rehabilitation after surgery for hip fracture. Patients were assessed by a physiotherapist at admission and discharge. Scale width, validity, minimal clinically important difference (MCID), responsiveness, and unidimensionality were investigated. Results: Evidence of convergent, discriminant and known groups validity were obtained for the DEMMI. Responsiveness was similar across instruments and the MCIDs were consistent with previous reports. A floor effect was identified for the 6 metre walk test and 6 minute walk test at hospital admission. Rasch analysis identified that the DEMMI maintains its unidimensional properties in this population. Conclusions: The DEMMI has a broader scale width than existing measures of activity limitation and provides a unidimensional measure of mobility for hip fracture patients during inpatient rehabilitation.