Objective: To evaluate the prevalence of genetic polymorphism incoagulation factors in thromboembolic disease in patients withclinical suspicion of thrombosis. Methods: A retrospective casecollection was performed searching all patients with clinicalsuspicion of thrombosis who were submitted to coagulation factoranalysis at the Hospital Israelita Albert Einstein from November2003 to April 2004. We included 260 patients, 118 male and 142female, mean age of 46 years. Prothrombin mutation and Leiden Vfactor were evaluated with multiplex PCR. Protein C, S and lupusanticoagulant were studied in coagulation assays. AntithrombinIII was studied by chromogenic assay. Anticardiolipin was evaluatedthrough an immunoenzymatic method and homocysteine by animmunometric method. Results: Factor V Leiden was found in 22cases (8.3 %), one homozygote and 21 heterozygotes. Prothrombinmutation was found in 18 cases (6.8%), one homozygote and 17heterozygotes. Forty cases with genetic alteration showed 34 caseswith thrombosis (85%), 29 venous thrombosis and 4 arterialthrombosis. One hundred and seven patients were tested foranticardiolipin antibodies: 21 were anticardiolipin antibody positives(19.6%), and 15 of them had IgG antibodies, 3 IgM and 3 IgA. Sixteenof the 21 patients had thrombosis, 11 venous thrombosis and 5arterial thrombosis. Lupus anticoagulant was found in two patients,both with thrombosis. There was only one case ofhyperhomocystinemia, with thrombosis. Protein C, protein S andantithrombin III deficiencies were found in 63 cases (12%). Out of31 cases with thrombosis (49,2 %), 26 cases had venous thrombosisand 5 had arterial thrombosis. Conclusion: Thromboembolicdisease is clearly associated with genetic factors but there isconsensus its cause is multifactorial. Genetic alterations, however,should be studied when there is clinical evidence of thrombosis,at least in young patients.