The relationship between quality of care and the cost of the nursing workforce is of concern to policymakers. This study assesses the evidence for a relationship between the nursing workforce and patient outcomes in the acute sector through a systematic review of international research produced since 1990 involving acute hospitals and adjusting for case mix. Twenty-two large studies of variable quality were included. They strongly suggest that higher nurse staffing and richer skill mix (especially of registered nurses) are associated with improved patient outcomes, although the effect size cannot be estimated reliably. The association appears to show diminishing marginal returns.