Although hospital work organization affects patient outcomes and in some states registered nurses (R.N.’s) are increasingly forming unions, the relationship between R.N. unions and patient outcomes has received little attention. This study examines the relationship between R.N. unionization and the mortality rate for acute myocardial infarction (AMI), or heart attack, in acute-care hospitals in California. After controlling for patient and hospital characteristics, the authors find that hospitals with unionized R.N.’s have 5.5% lower heartattack mortality than do non-union hospitals. This result remains substantively unchanged when the analysis accounts for possible selection bias—specifically, the possibility that unionized hospitals have certain important but unobservable characteristics, independent of unionization, that affect patient care.