Since most patients with thrombophilia in Israel are referred for diagnosis to our center, it was possible to estimate the relative frequency of the hereditary disorders leading to thrombophilia. 107 unrelated patients were evaluated over 4 years. Diagnoses were established in 23 patients (21.5%) while in 84 (78.5%) no abnormality was detected. Antithrombin III deficiency was found in 8 patients (7.5%), dominant protein C deficiency in 6 (5.6%), recessive homozygous protein C deficiency in 1, protein S deficiency in 3 (2.8%) and dysfibrinogenemia in 1. Four additional patients (3.7%) had a lupus anticoagulant. The frequency of deep vein thrombosis and pulmonary embolism was similar in patients with and without a definite diagnosis. Thrombosis of visceral or cerebral vessels and a positive family history were more frequent among patients in whom a definite diagnosis was made. In both groups there was a substantial lag between the time of presentation of the first thrombotic episode and the time of evaluation. Since the number of referred patients with thrombophilia has gradually increased over the period of the study, it is at present impossible to establish the prevalence of the various hereditary disorders leading to thrombophilia in the population.