Limited data exist on the impact of additional genetic risk factors on the clinical manifestations of factor (F) V Leiden homozygotes. A retrospective multi-centre cohort study was performed to assess the role of the FII G20210A gene mutation, the protein C (PC) promoter CG haplotype, the combination of two PC polymorphisms (A-1641G, C-1654T), the FXIII Val34Leu polymorphism, two thrombin-activatable fibrinolysis inhibitor polymorphisms (Thr325Ile, Ala147Thr), two plasminogen activator inhibitor-1 polymorphisms (-675 4G/5G, A-844G), the methylene-tetrahydrofolate reductase (MTHFR) C677T polymorphism and the ABO blood group on the thrombotic phenotype in FV Leiden homozygotes. 127 subjects with venous thrombosis and 53 asymptomatic subjects were analysed. The T allele of MTHFR C677T was more frequent in symptomatic subjects than in asymptomatic ones (68% vs. 45%, P = 0.02; odds ratio (OR) 2.8, 95% CI 1.3-5.8, after adjustment for potential confounders). For the other polymorphisms, no difference was observed between symptomatic and asymptomatic subjects. The non-O blood group was more frequent among symptomatic carriers (84% vs. 57%, P = 0.0002; OR 4.1, 95% CI 1.7-9.7). In conclusion, except for the ABO blood group, none of the polymorphisms studied contribute strongly to the thrombotic risk in FV Leiden homozygotes.