Background: Military training represents a significant physical challenge. Low fitness levels are strongly associated with an increased risk of injury and training attrition. To increase pass rates for Royal Navy (RN) Phase I training, a pre-joining fitness test (PJFT) was introduced (2.4 km treadmill best effort run). The PJFT was designed to identify candidates with poor levels of aerobic fitness who may be prone to leave training prematurely. Aim: To examine the impact of the PJFT on training length and outcomes. Methods: Time taken to run 2.4 km and training outcomes were measured before and after the introduction of the PJFT. Information was collected from RN Phase I training establishments and the network of careers offices between 2002 and 2005. Recruits were placed into quartile groups based on 2.4 km overground running performance. The no PJFT and PJFT groups and the 2.4-km run performance quartiles were compared for training outcomes and time spent in training. Results: Training measures were available for 4818 recruits who entered training before the PJFT's introduction (no PJFT) and 3305 after its introduction (PJFT). The pass rate increased from 78 to 88% following the introduction of the PJFT (P < 0.01). The number of recruits applying for voluntary release decreased from 15% (no PJFT group) to 6% (PJFT group) (P < 0.05). Conclusion: The PJFT positively impacted on RN Phase 1 training pass rates. A greater number of recruits successfully completed training, fewer applied for voluntary release and the number and length of training extensions were reduced.