RATIONALE: It is unclear whether elevated plasma fibrinogen associates with both deep venous thrombosis(DVT) and its complication pulmonary embolism(PE), and whether elevated fibrinogen is a direct cause of these disorders. OBJECTIVES: We tested the hypotheses that elevated plasma fibrinogen associate with increased risk of DVT alone, with any PE, and with PE in combination with DVT. METHODS: We studied 77,608 individuals from the Danish general population, of which 1,679 were diagnosed with DVT alone, 1,119 with any PE, and 272 were diagnosed with both PE and DVT. To test a potential causal relationship using a Mendelian randomization approach, we genotyped for FGB(rs1800790; rs4220) encoding fibrinogen beta chain. MEASUREMENTS AND MAIN RESULTS: Increasing plasma fibrinogen quintiles associated with increased risk of PE in combination with DVT(P-trend