Factor V Leiden (FVL) is an autosomal co-dominantly inherited Arg506-->Gly substitution of the activated protein C cleavage site affecting 5% of the Caucasian population. FVL results in impaired anticoagulant function without procoagulant modification. Heterozygotes experience a seven-fold increase in thrombotic events, whereas homozygotes may incur a 50 to 100 fold increase. Even though patients are at increased risk for deep venous thrombi, they experience a smaller risk of pulmonary embolism compared to individuals affected by other coagulopathies.