Regulations limiting duty hours for resident physicians were intended to improve safety, as resident fatigue has been linked to self-reported medical errors. Since the regulations were implemented, clinical outcomes have been unaffected, but it remains unclear if patient safety has improved. In this study, investigators performed root cause analyses on more than 100 cases of sentinel events to identify underlying causes, but did not find any cases where resident fatigue was a clear factor in the event. The Institute of Medicine recently issued recommendations for new duty hour regulations, and new regulations are expected to be implemented in 2011.