Venous thromboembolism (VTE) has been reported to be less common among Thais than Caucasians. Whether this observation reflects genetic or environmental factors, or both, is uncertain. To identify genetic and acquired risk factors of Thai patients with VTE, we enrolled in the study 105 consecutive Thai patients (34 men, 71 women) who had an objectively confirmed history of VTE. A complete clinical summary was obtained from each patient, with emphasis on personal and family history of VTE, as well as circumstantial vascular risk factors (surgery, immobilization, pregnancy, postpartum condition, trauma, oral contraceptive use, and malignancy). Of the 105 patients, 19% were found to have a malignancy. The mean age at the time of the first thrombotic episode was 52.1 years (range, 29-76 years), compared with 42.6 years (range, 17-82 years) for the patients without malignancy. Of the 85 patients without malignancy, 12.3% had protein S deficiency, 8.9% had protein C deficiency, 4.7% had antithrombin deficiency, 10.4% had antiphospholipid antibody, 30.4% had an elevated factor VIII level, 26.8% had an elevated factor XI level, 5.3% had hyperhomocysteinemia, and 16.5% were on oral contraceptives before the thrombotic episode. Factor V Leiden, the G20210A prothrombin gene mutation, and homozygosity for the C677T methylenetetrahydrofolate reductase (MTHFR) gene variant were not found. The VTE in 7.1% of the patients was considered to be secondary to recent surgery, trauma, and/or immobilization. Compared with studies of Caucasian patients, there were significant differences in the risk factors for VTE, with protein S deficiency and protein C deficiency being more common in the Thai patients. In contrast, factor V Leiden, the G20210A prothrombin gene mutation, and the C677T MTHFR gene mutation are not genetic risk factors among Thai patients with VTE. Malignancy and the use of oral contraceptives were the most common acquired risk factors for VTE in the Thai patients.