Hereditary labile factor (Factor V) deficiency is transmitted as an autosomal recessive. Only individuals who inherit the defective gene from both parents are bleeders. In the family studied, the father is Greek while the mother is Polish. The diagnosis is made by determining the prothrombin time by the one-stage method, which is completely corrected by the addition of freshly deprothrombinized plasma. Bleeding is mainly purpuric or capillary in type. Epistaxis is common, and menorrhagia is often serious. The important therapeutic agent is a transfusion of fresh plasma or fresh frozen plasma. Administration of vitamin K does not correct the prothrombin time and is ineffective clinically. Because of the hereditary character of the disease, genetic counseling may be helpful.