Background: Assessment of functional outcomes in survivors of severe injury is an identified priority for trauma systems. The predictive Functional Capacity Index (pFCI) within the 2008 Abbreviated Injury Scale dictionary (pFCI08) offers a widely available tool for predicting functional outcomes without requiring long-term follow-up. This study aimed to assess the 12-month functional outcome predictions of pFCI08 in a major trauma population, and to test the assumptions made by its developers to ensure population homogeneity. Methods: Patients with major trauma from Victoria, Australia, were followed up using routine telephone interviews. Assessment of survivors 12 months after injury included the Glasgow Outcome Scale – Extended (GOS-E). κ scores were used to measure agreement between pFCI08 and assessed GOS-E scores. Results: Of 20 098 patients with severe injury, 12 417 had both pFCI08 and GOS-E scoring available at 12 months. The quadratic weighted κ score across this population was 0·170; this increased to 0·244 in the subgroup of 1939 patients who met all pFCI assumptions. However, expanding the age range used in this group did not significantly affect κ scores until patients over the age of 70 years were included. Discussion: The pFCI08 has only a slight agreement with outcomes following major trauma. However, the age limits in the pFCI development assumptions are unnecessarily restrictive. The pFCI08 may be able to contribute to future systems predicting functional outcomes following severe injury, but is likely to explain only a small proportion of the variability in patient outcomes.