OBJECTIVES: Anticoagulants are widely used and represent a class of drugs that are problem-prone and have a high potential for adverse patient outcomes. As such, these drugs may be amenable to the use of prescribing guidelines. However, relatively little has been published on the effect of such guidelines on clinical outcomes or costs of care. The purpose of this study was to assess whether guidelines improve the appropriateness of prescribing, clinical outcomes, and the costs associated with use of anticoagulants in a sample of community hospitals in the United States. METHODS: A retrospective analysis was performed of data voluntarily collected by 15 hospitals before (July-September 2001) and after (March-May 2002) implementation of anticoagulant prescribing guidelines. Statistical analyses of both patient-level and hospital-level variables were conducted. RESULTS: Implementation of the guidelines resulted in a significant increase in the proportion of anticoagulants that were prescribed appropriately (59.8% v. 86.9%, p <0.001). The guidelines also resulted in a shift in the type of anticoagulants prescribed (decreased use of unfractionated heparin and increased use of low-molecular-weight heparins). There was suggestive evidence, though not statistically significant, that the guidelines resulted in fewer anticoagulant-associated adverse events (total bleeding RR = 0.71) and lower costs (savings of $56.15 per patient per day). CONCLUSIONS: While limitations existed with the study design, sufficient benefits were identified to warrant hospitals to consider use of these or similar guidelines on a routine basis. Clearly, additional study in this area would be useful.