The quality of hospital care varies widely so that one hospital may be more likely than another, even within one community, to save the life of a patient. The availability of nurses is a major determinant as to why outcomes differ among hospitalized patients. Establishing how exactly nurses affect patient safety and outcomes could ensure that local and national policies reflect the need for adequate nurse staffing. Researchers at the Center for Health Outcomes and Policy Research at the University of Pennsylvania are examining the ways that nurses influence patient outcomes using a measure of hospital performance called “failure to rescue.” One consistent finding is that lower patient-nurse ratios have been repeatedly linked to lower failure-to-rescue rates in hospitals. Because nurses in adequately staffed hospitals are better able to conduct patient surveillance, prepare for potential complications, and quickly take action when needed, patient outcomes in these hospitals are better. More hard data from smaller-scale studies of patient outcomes and studies of patients hospitalized for medical, not surgical reasons, will help to provide evidence that patient safety in hospitals hinges on the ability to recruit and retain sufficient numbers of qualified nurses, adequately supervise and mentor novice staff, and shape a supportive practice setting.