The University Medical Center Groningen is a level I trauma center in the northern part of the Netherlands. Sixty-three percent of all the patients admitted at the Trauma Nursing Department (TND) are acute patients who are admitted directly after trauma. In 2006 and 2007, the University Medical Center Groningen was not always capable of admitting all trauma patients to the TND due to the relatively high-bed occupation. Therefore, the reduction of the average length of stay (LOS) formed the objective of the project described in this study. We used the process-focused method of Lean Six Sigma to reduce hospital stay by improving the discharge procedure of patients in the care processes and eliminating waste and waiting time. We used the "Dutch Appropriateness Evaluation Protocol" to identify the possible causes of inappropriate hospital stay. The average LOS of trauma patients at the TND at the beginning of the project was 10.4 days. Thirty percent of the LOS was unnecessary. The main causes of the inappropriate hospital stay were delays in several areas. The implementation of the improvement plan reduced almost 50% of the inappropriate hospital stay, enabling the trauma center to admit almost all trauma patients to the TND. After the implementation of the improvements, the average LOS was 8.5 days. Our study shows that Lean Six Sigma is an effective method to reduce inappropriate hospital stay, thereby improving the quality and financial efficiency of trauma care.