Objective: This study prospectively evaluated in-hospital and postdischarge missed injury rates in admitted trauma patients, before and after the formalisation of a trauma tertiary survey (TTS) procedure. Methods: Prospective before-and-after cohort study. TTS were formalised in a single regional level II trauma hospital in November 2009. All multitrauma patients admitted between March-October 2009 (preformalisation of TTS) and December 2009-September 2010 (post-) were assessed for missed injury, classified into three types: Type I, in-hospital, (injury missed at initial assessment, detected within 24 h); Type II, in-hospital (detected in hospital after 24 h, missed at initial assessment and by TTS); Type III, postdischarge (detected after hospital discharge). Secondary outcome measures included TTS performance rates and functional outcomes at 1 and 6 months. Results: A total of 487 trauma patients were included (pre-: n = 235; post-: n = 252). In-hospital missed injury rate (Types I and II combined) was similar for both groups (3.8 vs. 4.8 %, P = 0.61), as were postdischarge missed injury rates (Type III) at 1 month (13.7 vs. 11.5 %, P = 0.43), and 6 months (3.8 vs. 3.3 %, P = 0.84) after discharge. TTS performance was substantially higher in the post-group (27 vs. 42 %, P < 0.001). Functional outcomes for both cohorts were similar at 1 and 6 months follow-up. Conclusions: This is the first study to evaluate missed injury rates after hospital discharge and demonstrated cumulative missed injury rates >15 %. Some of these injuries were clinically relevant. Although TTS performance was significantly improved by formalising the process (from 27 to 42 %), this did not decrease missed injury rates.