The meta-analysis included 28 studies, of which 17 were cohort studies, 7 were cross-sectional studies, and 4 were case–control studies (no experimental studies were identified). Most were U.S. studies, and the average level of staffing was 3.0 patients per RN for the intensive care unit (ICU) setting, 4.0 patients per RN in the surgical setting, and 4.4 patients per RN for the medical setting. It found a consistent relationship between higher RN staffing and lower hospital-related mortality: An increase of 1 RN full-time equivalent (FTE) per patient day was related to a 9% reduction in the odds of death in the ICU, a 16% reduction in the surgical setting, and a 6% reduction in the medical setting. With respect to other outcomes, lower rates of hospital-acquired pneumonia, pulmonary failure, unplanned extubation, failure to rescue, and nosocomial bloodstream infections were related to higher RN staffing in pooled analyses of several studies. However, several other outcomes that were presumed to have strong sensitivity to nurse staffing levels did not show consistent relationships, including falls, pressure ulcers, and urinary tract infections.