BACKGROUND AND OBJECTIVES: Factor V HR2 possesses decreased co-factor activity to activated protein C and an increased ratio of factor V1 to factor V2. Factor V HR2 is associated with a mild increase in the risk of venous thromboembolism although not all studies concur on this point. DESIGN AND METHODS: Inconsistencies in results of the epidemiological studies may stem from a failure to identify other variables in factor V which might contribute to an increased risk of thrombosis in selected HR2 carriers. The aim of this study was to establish whether factor V deficiency increases the risk of venous thromboembolism when associated with HR2. RESULTS: Four hundred and ninety-seven patients with venous thromboembolism and 498 controls were studied. HR2 was present in 12.5% of patients and 10.4% of controls. Factor V deficiency was associated with HR2 in 4.6% of patients and 1.0% of controls. The OR for venous thromboembolism in individual with HR2 alone was 1.2 (95% CI 0.8-1.8), while it was 4.7 (95% CI 1.8-12.5) for those with HR2 plus factor V deficiency. INTERPRETATION AND CONCLUSIONS: Patients with HR2 and factor V deficiency developed a thrombotic event earlier (median age 35 years) than patients with HR2 alone (median age 43 years, p = 0.018). Double heterozygosity for HR2 and a factor V defect, including factor V deficiency, increased the thrombotic risk afforded by HR2.