The review is devoted to the key component of plasma hemostasis — blood coagulation factor V. The structure of this protein and the F5 gene encoding it, its role in the hemostasis system, interaction with other coagulation factors and the natural anticulant protein C are considered. Particular attention is paid to the genetic defects of F5, which determine both hemorrhagic complications and a hereditary tendency to increased thrombus formation. Among the latter, the Leiden mutation of coagulation factor V (FV Leiden), which is hereditary thrombophilia and is considered as a risk factor for the development of venous thromboembolic complications, is described in detail.