Many registered nurses believe that nurse staffing in acute care hospitals is inadequate. In 1999 California became the first state to mandate minimum nurse-to-patient ratios in hospitals. State officials announced draft ratios in January 2002 and expect to implement the legislation by July 2003. We estimate that the direct costs of compliance will be small. However, mandatory ratios could generate opportunity costs that are not easily measured and that may outweigh their benefits. Policymakers elsewhere should consider other strategies to address nurses' concerns, because other approaches may be less costly and produce greater benefits to nurses and patients.