Oral anticoagulant therapy for patients who are at risk of developing blood clotting problems is used by between 400,000-600,000 Canadians annually. The use of this drug represents the most common cause of patient adverse medical outcomes due to medical errors. Furthermore, many patients have adverse outcomes using these drugs because physicians are not able to predict which patients are likely to have bleeding outcomes. Much effort has gone into developing ways to predict which patients are at risk of clotting but almost no work has gone into ways of predicting which patients would be at high risk of bleeding. This information is required to balance off the risk-benefits and to enable physicians and patients to understand the risks and benefits of taking these medications. Our study will develop a tool that can be used to predict bleeding risk in patients taking oral anticoagulant therapy. It will enable more informed decision making by both physicians and patients and will result in better control of the use of these drugs. In addition, patients who are at risk for being difficult to accurately dose on oral anticoagulants will be identified through our study.