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INTRODUCTION: Hereditary thrombophilia is caused by various inherited disorders which lead to familial tendency to recurrent venous thrombosis usually at an early age and with spontaneous onset. In the studies reported so far, the different prevalence of hereditary thrombophilia among patients with venous thrombosis was found, greatly depending on criteria for selection of patients. Arterial thrombosis is most often the consequence of arteriosclerosis but the prevalence of hereditary thrombophilia among young patients with arterial thrombosis and without recognized risk factors for arteriosclerosis is not known . In this study, the frequency of hereditary deficiencies of antithrombin III (AT III), protein C (PC), protein S (PS), plasminogen (PLMG), factor XII (F XII) and dysfibrinogenaemia was investigated over a 2-year period in 121 patients with venous or arterial thrombosis selected according to the recommendations of the British Committee for Standards in Haematology. PATIENTS AND METHODS: The study included total a of 121 patients (58 males and 63 females) with documented venous or arterial thrombosis. Table 1 shows patient's characteristics regarding gender, age and clinical manifestation of thrombosis. Each patient fulfilled at least one of the following criteria: a) venous thrombosis prior to the age of 45; b) arterial thrombosis prior to the age of 30, without risk factors for arteriosclerosis; c) recurrent thrombosis; d) familial tendency to thrombosis; e) thrombosis of unusual localization. A detailed history was taken from each patient on earlier personal or familial occurrence of thrombosis. For the purpose of this study, thrombophilia was characterized as congenital when the deficient protein was constantly below normal value and when the same deficiency was confirmed in a close family member; acquired when the acquired disorder predisposing to thrombosis was present in absence of constant protein deficiency; and idiopathic when the cause of thrombosis was unknown. All tests were performed in plasma obtained after centrifugation of venous blood anticoagulated with 0.129 mol/1 sodium citrate. Concentrations of fibrinogen, PT, PTT and F XII were measured by standard clotting methods. At III, PC and plasminogen activity were determined by chromogenic methods using commercial reagents (Boehring, Marburg, Germany). AT III, PC and total PS antigen were assayed by Laurell immunoelectrophoresis. The presence of lupus anticoagulant was investigated by recommended tests. RESULTS: A total of 15 patients (12.4%) fulfilled criteria for hereditary thrombophilia. Seven of them (5.8%) had AT III deficiency, five (4.1%) PC deficiency, two (1.6%) PS deficiency, and one patient had F XII deficiency. Secondary thrombophilia was found in 21.5% of patients and the cause of thrombosis in 66.1% of patients was not elucidated. A high frequency of hereditary thrombophilia has been found in patients with arterial thrombosis (40%). Among patients with hereditary thrombophilia thrombosis occurred at significantly younger age (29.9 vs. 42.2 and 40.9 yr.) compared to the patients with secondary and idiopathic thrombophilia, respectively. Patients with hereditary thrombophilia had also a higher occurrence of positive family history related to thrombosis (66.7% vs. 7.7% and 27.5%). DISCUSSION: The prevalence of hereditary thrombophilia in nonselected patients with venous thrombosis is relatively low, and for that reason the selection of patients, according recommended criteria, in whom the screening tests for congenital thrombophilia should be performed, is strongly suggested by many authors. In our study we used the generally accepted recommendations for investigation of patients with venous and arterial thrombosis. The presence of congenital thrombophilia was found in 15 (12.4%) of 121 studied patients, what is in accordance with results of other similarly designed studies. (ABSTRACT TRUNCATED)