Although recent research shows interesting relationships between hospital work organization and patient outcomes, the relationship between nurse unions and patient outcomes has not been explored. This study examines the relationship between the presence of a bargaining unit for registered nurses and the acute myocardial infarction (AMI) mortality rate for acute care hospitals in California. After accounting for patient and hospital characteristics, we find that hospitals with unionized RN workforces have 5.5 percent lower AMI mortality than do nonunion hospitals (-0.8 percentage point/14.6 percent average AMI mortality).