Only a minority of subjects with factor V (FV) Leiden mutation develop venous thromboembolism (VTE), suggesting that additional genetic risk factors may be present in symptomatic carriers. We screened 157 unrelated carriers of the FV Leiden mutation with a first episode of VTE and 291 unrelated asymptomatic FV carriers for the presence of two frequent mutations, i.e. G20210A of the prothrombin gene and C677T of the methylenetetrahydrofolate reductase gene. Carriers with other inherited or acquired thrombophilia-associated abnormalities were excluded from analysis. Heterozygotes for the G20210A mutation were more prevalent among symptomatic carriers than in asymptomatic carriers (10.8% v 2.7%, P<0.0001); homozygotes for the C677T mutation were also more prevalent in symptomatic carriers (21.6% v 14.4%, P = 0.05). Factor V Leiden carriers who had had a VTE episode during oral contraceptive intake were more frequently carriers of the G20210A mutation (14.3%, P = 0.03). These results further support the idea that VTE in carriers of FV Leiden results from interaction with additional genetic or circumstantial risk factors, and that an accurate search for such factors is required to identify carriers at risk.