peer reviewed ; The presence of the 2021 OA allele of the prothrombin gene has recently been shown to be a risk factor of venous thromboembolism, probably mediated through increased prothrombin levels. The aim of the study was to determine the frequency of the prothrombin 2021 OA allele in 193 consecutive unselected patients with venous thromboembolism and 100 healthy controls and to analyze the clinical profile associa!ed wi~h this new inherited thrombophilic factof. In agreement with previous reports, we found a frequency of 7.3% of heterozygous carriers of the 2021 OA allele among patients and 1% among controls. We confirm that plasma prothrombin levels are more elevated in the individuals bearing the prothrombin 20210A allele compared with those who do not. We did not find any relationship between the presence of the prothrombin 2021 OA allele and either a family history of thromboembolism, the rate of recurrences or the age at disease onset. However, the co-inheritance in the same individual of both prothrombin 2021 OA allele and factor V Leiden was associated with a significantly lower age at disease onset suggesting a synergistic contribution of both abnormalities.