Objectives: Thromboembolism is a serious medical problem causing considerable morbidity and mortality. Major clinical risk factors for thrombosis included surgery, fracture, malignancy, old age, immobilization and the use of oral contraceptives. In the last several decades, substantial progress has been made in identifying hereditary factors predisposing to thrombosis. The genetic defects known to be associated with thrombophilia are deficiencies of antithrombin 3, protein C, protein S, dysfibrinogenemia and resistance to the anticoagulant action of activated protein C. We have elucidated the characteristics of heriditary thrombophilia of the Korean patients. Methods: The clinical profiles of 48 patients with heriditary thrombophilia (12 cases of our hospital and 36 cases reported previously in Korea) were analyzed. The underlying hemostatic abnormalities about antithrombin 3, Protein C, Protein S, activated protein C, fibriongen were investigated. Family studies of 6 patients of our 12 patients were done. Nucleotide sequences of antithrombin 3 genes of 2 patients were studied. Results: 1) Seven patients (58%) among our 12 patients had thrombotic onset before fifth decades, and 5 patients developed thromboses at their third decades. 2) Pulmonary embolisms were diagnosed in 10 cases (83%) among our 12 cases. Deep vein thromboses of lower extremities were 8 cases (67%) and usually rare site thromboses like a portal vein thrombosis occurred frequently (8 cases, 67%). Arterial thromboses occurred in two cases (17%). Hereditability of 5 families were confirmed by family study. 3) Analysis of 48 cases showed that protein C deficiency (12 cases, 25%) and protein S deficiency (19 cases, 40%) occurred relatively frequently. 4) Neither Resistance to activated protein C nor mutation in the factor V gene was demonstrated in Korean patients with deep vein thrombosis and normal persons. This result suggests that activated protein C resistance may be extremely rare in Korean population. 5) Two new mutations of antithrombin 3 genes were identified in two patients via nucleotide sequencing, and they were named 'AT 3 Seoul' and 'AT 3 Kosung' respectively. Conclusion: We elucidated clinical and laboratory characteristics of hereditary thrombophilia in Korea. Hereditary thrombophilia were not uncommon in Korean patients with deep vein thrombosis, with/without pulmonary embolism. Strong suspicion for hereditary thrombophilia may lead to correct diagnosis and appropriate treatment in these patients.