Summary

The factor V Leiden (FVL) mutation, a genetic abnormality with an autosomal mode of inheritance, is associated with an increased risk of venous thromboembolism (VTE). We aimed to determine the annual incidence of VTE in first-degree relatives of patients with VTE and FVL and to identify factors in patients and the relatives that influence this incidence. In this retrospective and prospective cohort study, the incidence of objectively diagnosed first episodes of VTE was assessed in 553 first-degree relatives of 161 patients with acute VTE and FVL.The annual incidence of VTE was 0.43% (95% CI, 0.3 to 0.56) with FVL and 0.17 % (95% CI,0.07 to 0.27) without FVL (relative risk of 2.5,95% CI, 1.3 to 4.7). A majority (70%) of episodes ofVTE were provoked, and this proportion was similar with and without FVL.A larger proportion of VTE was provoked in women (83%) that in men (33%),with the difference accounted for by pregnancy and use of oral contraceptives.The proportion of pregnancies complicated by VTE was 3.9% (95% CI, 2.0–5.8) with FVL and 1.4% (95% CI, 0.04–2.7) without FVL. FVL is associated with a two- to three-fold increase in VTE in first-degree relatives of patients with VTE. No subgroup of relatives was identified who require more than routine prophylaxis because of a particularly high risk of VTE.