Background and Aims:?Hereditary thrombophilia (HT) is a condition increasing the risk to develop venous thrombosis. The prevalence of HT varies among different ethnic groups, but there are few studies of that in Korean patients. We aimed to estimate the frequency and clinical characteristics of HT in Korean patients with unprovoked VTE.?Methods:?A total of 369 consecutive patients with thromboembolic event who underwent thrombophilia tests were reviewed retrospectively. Arterial thromboembolism and VTE with predisposing factors were excluded. Patients with acquired thrombophilia were also excluded. Patients who had low levels of natural anticoagulants were assessed by DNA sequencing of the corresponding gene to confirm HT.?Results:?Among 222 Korean patients with unprovoked VTE, 66 patients were suspected HT. 62 of 66 underwent genetic molecular test and 33 were finally confirmed to have HT (53.2%; 33/62). The most common type was antithrombin III deficiency (14 of 222, 6.3%) followed by protein C deficiency (12, 5.4%), protein S deficiency (4, 1.8%) and dysplasminogenemia (3, 1.4%). Compared with patients without HT, those with HT were associated with higher proportion of male (69.7% vs 47%; p＝0.013), younger age (37 [32-50] vs 52 [43-65] years; p＝0.000), more history of VTE (57.6% vs 31.7%; p＝0.004) and family history (43.8% vs 1.9%; p＝0.000). Multivariate analysis showed age ＜45 years and presence of family history were strong predictor of unprovoked VTE with HT (odds ratio [OR]＝9.435; 95% confidence interval [CI]＝2.45-36.35; p＝0.001 and OR＝92.667; 95% CI＝14.95-574.29; p＝0.000, respectively). The cumulative incidence of recurrent VTE were not significantly different in both two group (p＝ 0.987), however all two events of recurrence in HT group have occurred under oral anticoagulant treatment.?Conclusion:?About 15% of patients with unprovoked VTE had HT, and antithrombin III deficiency was the most common cause. Above all unprovoked VTE patients who are less than 45 years old and have positive family history of VTE should be tested for HT. Furthermore, we need to closely monitor the recurrence of VTE despite maintaining anticoagulation, especially in patients with HT.