This retrospective study aimed to analyze laboratory findings in Thai patients with venous thrombosis in Phramongkutklao Hospital from August 1997 to October 2004. Blood samples obtained from 166 patients with ages ranging from 10 months to 87 years were tested for protein S (PS), protein C (PC), antithrombin (AT), factor V Leiden (FVL) and prothrombin G20210A. It was found that low levels of PS, PC, and AT were observed in 23 patients (13.9%), 21 patients (12.7%) and 11 patients (6.6%), respectively. The incidence of combined low levels of anticoagulant factors occurred in 23 patients (13.9%). Three patients (1.8%) were positive for FVL. All patients were negative for prothrombin G20210A. Additionally, 85 patients (51.2%) were negative for all tests. In conclusion, it is recommended that the screening tests for anticoagulant factors PS, PC and AT be used to investigate the causes of thrombosis in Asian populations due to their cost-effectiveness. However, the detection of gene mutations inducing thrombosis should be considered.