Factor V Leiden is the most common hereditary blood coagulation disorder. It is present in 3-8% of a healthy white population and it is associated with venous thrombosis especially in oral contraceptive users. The author evaluates a proposal to introduce screening for Factor V Leiden mutation in oral contraceptive users who have a first degree relative who has suffered venous thrombosis, and deter test positives from oral contraception, in comparison with standard practice of deterring on the basis of family history only.