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High factor VIII levels increase the risk of venous thromboembolism, but the mechanisms that cause high factor VIII levels are unclear. In 301 thrombosis patients and 301 matched healthy controls, factor VIII antigen (VIII:Ag) levels > or = 150 IU/dl increased the thrombosis risk more than fivefold. We investigated whether high factor VIII:Ag levels result from a genetic variation in the factor VIII or von Willebrand factor (VWF) genes. Six polymorphisms in the VWF gene and two CA-repeats in the factor VIII gene were not associated with plasma VWF levels, factor VIII:Ag levels, or thrombosis risk. Our data do not support the hypothesis that a single functional sequence variation in the factor VIII or VWF gene explains the majority of high factor VIII levels and thrombotic risk.