Mild hyperhomocysteinemia has been shown to be a risk factor for venous thrombosis and may be caused by genetic and/or environmental factors. The most common genetic variant that can result in mild hyperhomocysteinemia is caused by thermolabile variant of the enzyme C677T methylenetetrahydrofolate reductase (MTHFR). The frequency of the homozygous variant of MTHFR genotype was observed in range 10 to 15 % and the heterozygosity for variant is more frequent in these subjects in about 45% of the European population. The prevalence of a mutation in the MTHFR gene is slightly higher in patients with venous thromboembolism. The frequency of MTHFR was assessed in a group of 115 consecutive patients (from 115 unrelated families) with first or recurrent episode of venous thrombosis under the age of 50. The authors confirmed high prevalence (heterozygotes constituted 41.7% and homozygous 11.4%) of this mutation in young thrombophilic patients in accordance with similar cohorts. MTHFR variant is not a risk factor for thrombosis itself. The authors continue this study by measurement of fasting homocysteine and acid folic levels, with the aim of identification of subgroup patients with MTHFR mutation who have the risk of venous thromboembolism. Key words: venous thromboembolism, mild hyperhomocysteinemia, thermolabile variant of the C677T methylenetetrahydrofolate reductase, young patients

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