BACKGROUND: The Health Assessment Questionnaire-Disability Index (HAQ-DI) is used to assess functional status in rheumatoid arthritis (RA), but the change required for meaningful improvements remains unclear. A minimum clinically important difference (MCID) of 0.22 is frequently used in RA trials. The aim of this study was to determine a statistically defined critical difference for HAQ-DI (HAQ-DI-d) and evaluate its association with therapeutic outcomes. METHODS: We retrospectively analyzed data from adult German patients with RA enrolled in a multicenter observational trial in which they received adalimumab therapy at the decision of the treating clinician during routine clinical care. The HAQ-DI-d, defined as the minimum change that can be reliably discriminated from random long-term variations in patients on stable therapy, was determined by evaluating intra-individual variation in patient scores. Other outcomes of interest included Disease Activity Score-28 joints and patient-reported pain and fatigue. RESULTS: The HAQ-DI-d was calculated as an improvement (decrease) from baseline of 0.68 in a discovery cohort ( = 1645) of RA patients on stable therapy and with moderate disease activity (mean DAS28 [standard deviation] of 4.4 [1.6]). In the full patient cohort ( = 2740), 22.1% of patients achieved a HAQ-DI-d improvement at month 6. Compared with patients with a small improvement in HAQ-DI (decrease of ≥0.22 to < 0.68) or no improvement (< 0.22), patients achieving a HAQ-DI-d at month 6 had better therapeutic outcomes at months 12 and 24, including stable functional improvements. Change in pain was the most important predictor of HAQ-DI improvement during the first 6 months of therapy. CONCLUSIONS: A HAQ-DI-d of 0.68 is a reliable measure of functional improvement. This measure may be useful in routine clinical care and clinical trials. TRIAL REGISTRATION: ClinicalTrials.gov NCT01076205. Registered on February 26, 2010 (retrospectively registered).