The aim of the study was to determine the frequency of additional prothrombotic defect in family members of 14 symptomatic, heterozygous carriers of factor V Leiden mutation (FV Leiden). The FV Leiden was found in fifty-five from among 127 persons (43%). Thirty-two from 53 (68%) family carriers of FV Leiden had venous thromboembolic disease at the time of examination. Combined defects were found in 6 out of 14 families (in 4 families coexistence of FV Leiden mutation with protein S deficiency, in 1 with G20210 A mutation of prothrombin gene and in 1 with antithrombin III deficiency). Until now 63% (7/11) of combined defect carriers (mean age 48.5 years) and 59% (24/42) of heterozygous FV Leiden carriers (mean age 52.2 years) have been symptomatic. The thrombosis-free survival (Kaplan-Maier analysis) was comparable in combined and in single defect carriers groups.