The prevalences of deficiencies in antithrombin III (AT III), protein C (PC), protein S (PS) and in the activated protein C (APC) resistance in the thrombotic population of the Trakya region, Turkey were investigated. 37 patients with venous thrombosis (VT) and 17 patients with arterial thrombosis (ArT) were included in this study. The mean ages of the patients with VT and ArT were 46 years (range 20-70) and 38 years (range 32-40), respectively. The activity of AT III was measured by commercially available immuno-turbidimetric assay. The activities of PC and PS were determined by coagulometric assay. The APC resistance was measured using a modified APTT-based clotting assay. Among the VT patients, there were 2 cases (5.4%) with AT III, 5 (13.51%) with PC deficiency, 5 (13.51%) with PS deficiency and 2 (5.4%) with APC resistance. In the ArT patient group, there was 1 patient (5.88%) with AT III, 3 (17.64%) with PC deficiency, 1 (5.88%) with PS deficiency and no APC resistant patients, while there was one (2.08%) with PC deficiency and one (2.08%) with APC resistance in the control group (49 persons, mean age 41 years). The relative risk of thrombosis (odds ratio) was 1.7 in the deficiency of PC and 5.6 in the deficiency of PS. The data presented suggests that the prevalences of AT III, PC and PS deficiencies causing thrombophilia in the Trakya region of Turkey are higher than in other reported studies while the APC resistance is lower than in others. Further studies including more patients would be required to clarify these discrepancies.