The risk of hemorrhage when using coumarin anticoagulants sharply increases when the International Normalized Ratio (INR) is > or =6.0. We performed a case-control study among outpatients of an anticoagulation clinic to identify sociodemographic-, lifestyle-, and dietary factors related to overanticoagulation. Three hundred cases with an INR > or =6.0 were compared with 302 randomly selected matched controls with an INR within the target zone. Age, sex, and level of education were not associated with overanticoagulation. Body mass index was negatively related to overanticoagulation, a beneath-average level of physical activity was positively related to overanticoagulation and never-smokers were more likely to have an INR > or =6.0 compared with smokers. Habitual alcohol consumption, even heavy drinking, was not related to overanticoagulation. However, a recent decrease of alcohol intake increased the risk of an INR > or =6.0. In addition, weight loss and a vacation were risk factors for overanticoagulation. Dietary factors were not associated with overanticoaguation. If risk factors can not be avoided, increased monitoring of INR values could prevent overanticoagulation and potential bleeding complications.