Abstract
An Australian family with familial antithrombin Ill (AT III) deficiency is described. The deficiency inherited in an autosomal co‐dominant manner is characterised by proportionate reduction in antigenically and biologically measured AT III. Some members with AT Ill deficiency have had major venous thromboses, and the deficiency has possibly been the cause of death in two individuals in the family. Heterogeneity was observed in laboratory and clinical findings in this family.