To discuss the advantages of different methods of determining the effectiveness of healthcare interventions, to evaluate the basis for the assumption that right heart catheterization is effective, and to consider how right heart catheterization may fail to improve outcomes in some circumstances.Published English language literature on right heart catheterization and patient outcomes.We recently reported an association between the use of right heart catheterization in the initial care of critically ill patients and increased risk of death. This finding is troubling and warrants future study. In this article I address the role of randomized controlled trials and observational studies in the evaluation of the effectiveness of healthcare interventions. I discuss the basis in the published literature for assuming that right heart catheterization improves outcomes. Finally I present a variety of problems with right heart catheterization which might allow this procedure, which logically should help patients, to fail to achieve a beneficial effect for some patients.We must do appropriate prospective studies to determine who benefits from right heart catheterization and who does not. We owe our patients nothing less.