Recently a new identified genetic variant in the 3'-untranslated region of the prothrombin gene (G20210A allele) associated with increased plasma prothrombin levels has been linked to an increased risk of venous thromboembolism (VTE). To determine the importance of this G20210A allele as a causative risk factor for VTE in the general population, we analysed the data of an epidemiologic investigation on thrombophilia in young people, the Oporto Thrombophilia Study (PORtromb). Forty cases (mean age: 27 yr old) with a first episode of VTE under 40 yr old, and 100 healthy subjects, were evaluated. Heterozygosity for the G20210A allele was present in 12.5% of VTE cases and in 5% of controls, indicating a possible marginal increase of VTE risk in carriers of the allele (odds ratio: 2.71: 95% CI 0.74-9.95).