In family studies, the risk for venous thromboembolism (VTE) in relatives with factor V Leiden (FVL) or G20210A prothrombin (PT20210A) gene polymorphisms may differ according to genotype and clinical presentation of the proband. To address this hypothesis, a retrospective cohort family study was carried out on 192 kindreds with at least one member with homozygous FVL or PT20210A, for a total of 886 relatives. The proband of the family was heterozygous in 68 and homozygous or with both polymorphisms in 124 kindreds. Twenty-three probands were asymptomatic, 11 had had arterial thrombosis, 7 obstetrical complications, and 151 venous thrombosis (122 VTE and 29 superficial vein thrombosis). The incidence of VTE (per 1000 patient-years) in relatives was higher when the proband had heterozygous rather than homozygous polymorphism (1.25 [95% confidence interval (CI), 0.73-1.91] vs 0.44 [0.19-0.78]) and when the proband had had VTE instead of other or no clinical manifestations (0.95 [0.57-1.42] vs 0.50 [0.19-0.96]). Compared with relatives belonging to kindreds with homozygous probands without VTE, the adjusted hazard ratio of VTE for relatives selected from kindreds with heterozygous probands with VTE was 4.14 (95% CI, 1.17-14.71). The genotype and clinical presentation of the proband influence the risk for VTE in relatives with FVL or PT20210A.