We evaluate the impact of California Assembly Bill 394, which mandated maximum levels of patients per nurse in the hospital setting. When the law was passed, some hospitals already met the requirements, while others did not. Thus changes in staffing ratios from the pre- to post-mandate periods are driven in part by the legislation. We find persuasive evidence that AB394 had the intended effect of decreasing patient/nurse ratios in hospitals that previously did not meet mandated standards. However, these improvements in staffing ratios do not appear to be associated with relative improvements in measured patient safety in affected hospitals. (C) 2012 Elsevier B.V. All rights reserved.