55 consecutive patients (28 males and 27 females) who had suffered from (recurrent) venous and/or arterial thromboembolism were evaluated for laboratory findings of thrombophilia. At the time of investigation, 15 patients were taking oral anticoagulants. In addition to patient and family history and clinical examination, a coagulation profile, euglobulin clot lysis time before and after venous occlusion, assays of plasminogen, antithrombin III, protein C (PC), free protein S (PS[f]), and C4b binding protein bound protein S (PS[b]) were obtained. 2 patients not orally anticoagulated had partial PD deficiency with functional PC activity values of 53% and 57% respectively, as compared to normal human plasma (NHP). Functionally active PS(f) was found to be lower than normal in 14 patients not taking oral anticoagulants and without signs of hepatopathy. In 4 of these 14 patients partial PS deficiency was present (PS[f] between 7 and 37% of NHP, and PS[b] between 51 and 86% of NHP). In one of these 4 subjects the hereditary nature of PS deficiency was proven by investigation of family members. In 3 of the 14 patients a shift from PS(f) (between 34 and 66%) towards PS(b) (124-141%) was found. The remaining 7 patients showed subnormal PS(f) values (56-64%) and normal PS(b) values (78-105%). In the patients receiving oral anticoagulant therapy, PC and PS levels were diminished. Statistically there was no close relationship between PC and PS levels on the one hand and prothrombin time values on the other. In anticoagulated patients, therefore, PC or PS deficiency can only be diagnosed by investigation of members of the propositus's family.(ABSTRACT TRUNCATED AT 250 WORDS)