OBJECTIVE: To examine the intertester reliability of the three activities of the Cumulated Ambulation Score (CAS) and the total CAS, and to define limits for the smallest change in basic mobility that indicates a real change in patients with hip fracture. DESIGN: An intertester reliability study. SETTING: An acute 20-bed orthopaedic hip fracture unit. SUBJECTS: Fifty consecutive patients with a median age of 83 (25-75% quartile, 68-86) years. INTERVENTIONS: The CAS, which describes the patient's independency in three activities - (1) getting in and out of bed, (2) sit to stand from a chair, and (3) walking ability - was assessed by two independent physiotherapists at postoperative median day 3. Each activity was assessed on a three-point ordinal scale from 0 (not able to) to 2 (independent of human assistance). The cumulated score for each activity provides a total CAS from 0 to 6, with 6 indicating independent ambulation. MAIN MEASURES: Reliability was evaluated using weighted kappa statistics, the standard error of measurement (SEM) and the smallest real difference (SRD). RESULTS: The kappa coefficient, the SEM and the SRD in the three activities and the total CAS were >or=0.92,