Disability following trauma has profound personal, social and economic costs. Currently, measurement of disability is not standardised and no agreed time interval between injury and disability measurement exists. This study was designed to define the best time at which to measure disability following trauma. The functional independence measurement (FIM) and Glasgow outcome scale (GOS) were assessed at 3, 6, 12 and more than 24 months after injury for 201 trauma patients. The best time to measure was defined as the point at which a steady state was reached, i.e. when further functional improvement ceased. Motor FIM showed significant change between 3 and 6 month assessments (p < 0.002) and 6 and 12 months (p < 0.002). No statistically significant change occurred beyond 12 months (p > 0.02). For Cognitive FIM, there was significant change between 3 and 6 months (p = 0.02), but not beyond 6 months (p > 0.2). For GOS there was significant change between 3 and 6 months (p < 0.002) and 6 and 12 months (p < 0.002) but not beyond 12 months (p > 0.2). Disability measurements should be performed 12 months after injury, when patients have reached a steady state. This time of measurement should be adopted as the standard for trauma databases and outcome studies.