A small percentage of patients die during hospitalization or shortly thereafter, and it is widely believed that more or better nursing care could prevent some of these deaths. The author systematically reviewed the evidence about nurse staffing ratios and in-hospital death through September 2012. From 550 titles, 87 articles were reviewed and 15 new studies that augmented the 2 existing reviews were selected. The strongest evidence supporting a causal relationship between higher nurse staffing levels and decreased inpatient mortality comes from a longitudinal study in a single hospital that carefully accounted for nurse staffing and patient comorbid conditions and a meta-analysis that found a "dose-response relationship" in observational studies of nurse staffing and death. No studies reported any serious harms associated with an increase in nurse staffing. Limiting any stronger conclusions is the lack of an evaluation of an intervention to increase nurse staffing ratios. The formal costs of increasing the nurse-patient ratio cannot be calculated because there has been no evaluation of an intentional change in nurse staffing to improve patient outcomes.