Consecutive patients with a history of thrombo-embolic disease (n = 241, 109 males, 132 females, mean age 46 y), referred to the Coagulation Laboratory during an 18 month period, were analysed for defects in their coagulation and fibrinolytic systems. The diagnosis of thrombosis had been verified with phlebography and that of pulmonary embolus with scintigraphy or angiography. Retinal venous thrombosis was found in 15 of the patients. In 15 cases the thrombotic episodes occurred postoperatively, in 15 during pregnancy, in 12 during the postpartum period and in 20 during use of oral contraceptives. In the remaining cases no clinical riskfactors were identified.The concentration of protein C zymogen was measured with an immunoradiometric assay. Functional protein C was determined with a clotting inhibition assay. Protein C deficiency was found in 8 cases. Two of these had a functional protein C deficiency with normal zymogen levels. The concentration of total, as well as free (not in complex with C4b-binding protein), protein S was determined with a radioimmunoassay. Two cases of protein S deficiency were detected. Three patients with antithrombin III deficiency and two with plasminogen deficiency were found.The fibrinolytic activity after venous occlusion was analysed in 216 patients. Decreased levels were found in 32 %. The concentration of tissue plasminogen activator inhibitor (PAI) was measured in 110 patients and found to be increased in 65 % of the cases. In 99 patients both the fibrinolytic activity and the PAI concentration were measured. A combination of decreased fibrinolytic activity and increased levels of PAI was found in 44 cases. The concentration of tissue plasminogen activator antigen was decreased in 22 % of 105 cases analysed.Thus, in this material of patients with thrombo-embolic disease, abnormalities were found in 47 %. Defects in the fibrinolytic system were the most common findings. Protein C or protein S deficiency was diagnosed in less than 5 % of the cases.