Factor V Leiden (FVL) mutation and prothrombin G20210A mutation are common hereditary risk factors for venous thrombosis. In the current study, 40 patients (mean age +/- standard deviation, 35 +/- 11 years) and 764 healthy control subjects (mean age +/- standard deviation, 37 +/- 14 years) were enrolled to assess the potential role of these mutations in the manifestation of thrombotic microangiopathies. Compared with controls, neither the heterozygous FVL mutation (7.5% vs 8.5%; P = 1) nor the heterozygous prothrombin mutation (2.5% vs 2.8%; P = 1) was more prevalent in the patients. The findings do not support a significant role of FVL and prothrombin mutations as risk factors for the manifestation of thrombotic microangiopathies. Thus, screening for these mutations does not allow the identification of individuals at increased risk for these rare thrombotic disorders.