Several studies have pointed to a relationship between nurse staffing ratios (particularly staffing by RNs) and patient safety. Such studies have influenced public policy. For example, the state of California mandates certain minimum nurse-to-patient ratios in intensive care units and medical–surgical units. However, methodological limitations have led some to question the strength of this association. This study, the most methodologically robust to date, identified a statistically significant relationship between higher RN staffing and lower patient mortality. High patient turnover was also associated with excess mortality. The study is likely to increase the pressure on hospitals to ensure adequate levels of nurse staffing.