The G20210A mutation of the prothrombin (PT) gene has recently been identified as a risk factor for venous thromboembolism (VTE). This mutation was shown to be present mainly among Caucasian populations, with a higher frequency in southern than in northern Europe. The aim of our study was to determine the prevalence of the PT 20210A allele in the Polish general population and in patients with a history of venous thrombosis. The patient group comprised 323 subjects with VTE before the age of 45, recurrent VTE or thrombosis in an unusual site. The control group consisted of 399 healthy individuals. Heterozygosity for the PT 20210A allele was found in 21 (6.5%) patients and 7 (1.8%) controls. In 7 (33.3%) of the 21 heterozygous patients the PT 20210A allele was associated with the factor V Leiden mutation, in 1--with the homozygous C677T mutation of the methylenetetrahydrofolate reductase (MTHFR), and in 1--with lupus anticoagulant. Our results indicate that the presence of the 20210A allele is a mild risk factor for venous thrombosis if not associated with other thrombophilic defect (odds ratio 2.2; 95% CI: 0.8-5.5). The risk is greater in double heterozygous carriers of the PT 20210A allele and factor V Leiden mutation.