BACKGROUND Factor V Leiden (FVL) is the most common heritable cause of venous thrombosis. It is caused by a single nucleotide substitution resulting in an R506Q missense mutation, resulting in factor V resistance to activated protein C (APC) inactivation. Carriers of FVL have an increased susceptibility to venous thrombosis, which is further increased in the presence of other genetic or environmental risk factors. OBJECTIVE To review the biology, clinical findings, laboratory detection methods, and screening recommendations for patients with the FVL mutation. METHODS PubMed review of published literature and online information. CONCLUSIONS FVL remains an important heritable cause of hypercoagulability since its discovery more than 10 years ago. Clinical suspicion should be high in cases of unexplained venous thrombosis. APC resistance and FVL mutation can be diagnosed with high sensitivity and specificity with use of clotting time-based functional assays and genetic assays, respectively, allowing for evidence-guided clinical decision making regarding the benefit of long-term anticoagulation.