Laboratory Studies All laboratory evaluations will be carried out at least 2 weeks after cessation of the anticoagulant therapy or after a switching to low-molecular weight heparin (LMWH) treatment. Antithrombin (AT) functional activity will be measured and VTE patients will be stratified into 3 subgroups of AT% (1st: <70%, 2nd: 70-80, 3rd: >81%). AT measurements will be confirmed in a second sample collected 3 months later. In case of discrepancy between the results of the 2 tests, a third sample will be collected. Protein C and S; antiphospholipid antibodies (IgG and IgM); homocysteine, and factor V Leiden and prothrombin 20210A polymorphisms will be determined using commercially available kits by technicians who had been unaware of the patients' treatment assignments and of the clinical course.