Life-long symptoms of venous thromboembolism were present throughout five generations in a large Italian kindred. Eight out of the 22 tested members of the family had reduced plasma levels of antithrombin III (AT III), the main physiological inhibitor of blood coagulation. Three of them, aged 2-28 years, but no thrombotic manifestations. AT III deficiency, which was hitherto thought to be very rare in this country, appeared to be related to a reduced synthesis of the protein without any qualitative abnormality. This view is supported by the concomitant reduction of AT III levels as demonstrated by biological and immunological assays, as well as by a normal pattern of the protein in double-crossed immunoelectrophoresis with and without heparin added to the agarose gel. Long-term treatment with oral anticoagulants appeared to prevent further thromboembolic episodes in two patients, even though plasma AT III levels remained unchanged.