In a prospective cohort study, we assessed the incidence of spontaneous and risk period-related venous thromboembolism (VTE) in asymptomatic family members of patients who experienced VTE and had the factor V Leiden mutation. In all, 561 family members of 131 probands were included, 313 of whom were carriers (299 heterozygous and 14 homozygous) and 248 of whom were noncarriers of the factor V Leiden mutation. Average follow-up was 4 years (range, 4 months-6 years). There were 1255 and 984 observation-years of follow-up in carriers and noncarriers, respectively. Eight episodes of VTE occurred in heterozygous carriers, resulting in an annual incidence of 0.67% (95% confidence interval [CI], 0.29-1.33). Two events occurred in the absence of associated risk factors, determining an annual incidence of spontaneous VTE of 0.17% (95% CI, 0.02-0.6). Only one VTE (risk period-related) occurred in noncarriers, with an annual incidence of 0.1% (95% CI, 0.003-0.56). Relative risk for VTE in heterozygous carriers compared with noncarriers of the factor V Leiden mutation was 6.6 (95% CI, 1.1-39.8). Risk period-related VTE occurred with an incidence of 18% and 5% per risk period in heterozygous carriers and in noncarriers, respectively. Thus, the low rate of VTE in asymptomatic family members carrying the mutation did not justify continuous anticoagulant prophylaxis. Screening families of symptomatic probands with the factor V Leiden mutation has the potential to identify those asymptomatic carriers who might benefit from thromboprophylaxis during risk periods.