Warfarin is currently prescribed to over 2 million Americans. It is perhaps the most hazardous drug commonly prescribed today. Dosing errors causing excessively high or low anticoagulant effects are common, particularly during the first month of treatment. Sources of correctable dosing errors have been identified and include poor dosing judgement, failure to recognize well-described drug interactions, and failure to communicate important dosing information during the transition between hospital and outpatient follow-up. The overall objective of this proposal is to advance patient safety by creating clinical informatics tools specifically designed to reduce the incidence of serious warfarin dosing errors in hospitalized patients being starting on warfarin for the first time. Our specific aims are to : 1) modify and enhance for easy use on a pocket PC computer a Bayesian forecasting program that predicts warfarin dose requirements, and couple this with a specially written program that identifies important drug interactions; 2) create for outpatient providers an Anticoagulation Discharge Summary Report that contains the important information necessary to make proper warfarin dosing after hospital discharge; and 3) conduct a large multi-centered clinical trial evaluating the effectiveness of their combined use in reducing warfarin dosing. The primary outcome will be incidence of an excessively high anticoagulant. However, in order to determine the extent to which use of these informatics tools leads to a more sustained impact after hospital discharge, we will quantify quantifying the incidence of thromboembolic or bleeding complications for 4 weeks after discharge. We will also evaluate the feasibility of using these informatics tools in routine hospital practice. After documenting the effectiveness of these informatics tools, they will be made open source and widely available for use by the public by placing them on a variety of web sites for easy downloading.