Antithrombin III is the major physiological inhibitor of the coagulation mechanism and a deficiency of this protein results in a marked predisposition to venous thromboembolic disease. Three Scottish families with a deficiency of this protein are described and other reported families are reviewed. The properties, functions and methods of assay of antithrombin III are outlined; the molecular abnormalities, inheritance, clinical and laboratory characteristics of antithrombin III deficiency are described, and the use of antithrombotic drugs and human antithrombin III concentrates in this deficiency is discussed.