An antithrombin III (AT III) functional defect (AT III Charleville) was discovered in a patient presenting with recurrent venous thrombosis. Both anti-activated factor X (anti Xa) and antithrombin activity were decreased, in the absence and in the presence of heparin, while protein concentration was normal in an immunological assay. The abnormal AT III copurified with functional AT III using insolubilized heparin affinity chromatography. Polyacrylamide gel electrophoresis (PAGE) and high pressure liquid chromatography (HPLC) on a TSK column suggest that AT III Charleville forms unstable complexes with thrombin from which a modified protein is rapidly released.