The aim of the study was to evaluate the thrombosis free survival in unrelated 24 heterozygous carriers for Leiden mutation and in unrelated 26 patients with "idiopathic" thromboembolic disease. The subjects with coexisting antithrombin III, protein C, S deficiency, obesity, antiphospholipid antibodies, cancer or taking oral contraceptives were excluded from analysis. The clinical manifestations of thrombosis were comparable in both investigated group, similarly the mean age of the first episode occurrence (37.2 vs 41.5 years). The probability for development of thrombosis in Kaplan-Meier analysis was not different between two groups. Moreover, during six months of observation period thrombosis reoccurrence rate was similar (1/24 vs 1/26).