In order to assess whether the HR2 haplotype of the factor V gene (HR2) increases the risk of venous thromboembolism (VTE) in carriers of antithrombin (AT), protein C (PC) or S (PS) defects, we performed this determination in 336 subjects, who were family members of 66 symptomatic patients with clotting inhibitors defects. We first assessed the presence of previous VTE, and then followed prospectively subjects without prior VTE. VTE episodes had occurred in 26 individuals: 18 in 139 carriers of clotting inhibitors defects alone (annual incidence, 0.55%), four in 33 carriers of clotting inhibitors defects combined with HR2 (0.52%) and four in 151 non-carriers (0.1%), resulting in a relative risk (RR) for VTE of 4.9 (95% CI: 1.7-14.4) and 4.62 (95% CI: 1.2-18.4), respectively. After an overall follow-up of 2557 patient-years, VTE episodes developed in 12 subjects: nine in 121 carriers of clotting inhibitors defects alone (annual incidence, 0.92%), three in 29 carriers of clotting inhibitors defects combined with HR2 (1.0%) and none in 147 non-carriers. In family members of patients with AT, PC or PS defects the coinheritance of HR2 haplotype does not seem to increase the thromboembolic risk.