BACKGROUND: The de Morton Mobility Index (DEMMI) is an instrument that accurately measures the mobility of older people across clinical settings. PURPOSE: To report the multiple reliability studies conducted during the development and validation of the DEMMI. METHODS: Intra-rater and inter-rater reliability studies were conducted for the DEMMI in two independent samples (development and validation samples) of older acute medical patients (aged 65 years or older). Inter-rater reliability studies were conducted between the test developer (a physiotherapist) and another experienced physiotherapist. Order of assessor administration was randomized by a coin toss. Patients who were fatigued after the first assessment were excluded from the inter-rater reliability study. Intra-rater reliability studies included participants with 'unchanged' mobility status between hospital admission and discharge. Scale reliability estimates were expressed as the minimal detectable change with 90% confidence (MDC90 ). Item reliability was calculated using Kappa statistics and absolute percentage agreement. RESULTS: The MDC90 for the DEMMI development sample was 9.51 points (95% confidence interval [95% CI], 5.04-13.32; n=21) and 7.84 (95% CI, 4.34-11.65; n=16) on the 100-point interval DEMMI scale for the inter-rater and intra-rater reliability studies, respectively. Similar estimates were obtained for the DEMMI validation samples of 8.90 (95% CI, 6.34-12.69; n=35) and 13.28 points (95% CI, 8.08-20.87; n=19). Items were not excluded from the DEMMI based on the results of item reliability. CONCLUSION: Reliability estimates for the DEMMI were consistent across independent samples of older acute medical patients using different reliability study methodology. Error represents approximately 9% of the DEMMI scale width.