The 34 Leu (100T) variant of the factor XIII Val34Leu (G100T-) polymorphism slows down fibrinolysis and has been proposed as a thrombotic risk factor. In this pilot study, we enrolled 40 patients (mean age +/- SD = 38 +/- 11 years) and 728 controls to assess the role of this genetic variant for the manifestation of thrombotic microangiopathies. From the genotype prevalences, an increased manifestation risk for carriers of the TT genotype (homozygous Leu variant) of the factor XIII Val34Leu (G100T-) polymorphism was calculated (odds ratio [OR] = 2.44; 95% confidence interval [CI] = 0.8-7.6; P = .11). This association was statistically significant for patients with thrombotic thrombocytopenic purpura-hemolytic-uremic syndrome (TTP-HUS) (OR = 6.6; 95% CI = 1.7-25.9; P = .006). Our data suggest a role of the homozygous Leu variant of the factor XIII Val34Leu polymorphism in the manifestation of thrombotic microangiopathies. Decreased fibrinolysis in the presence of this genetic variant provides a plausible explanation for this association.