Data on clinical features and laboratory diagnosis of familial antithrombin deficiency, a rather heterogeneous group of disorders, are illustrated by observations on two Romanian kindreds afflicted by recurrent thrombotic episodes. In a first family, both plasma antithrombin III antigen and activity were reduced to 50% of normal, a condition characteristic for a heterozygous type I (quantitative) familial antithrombin III deficiency. In a second kindred, the two brothers who had experienced thrombotic events since they were teenagers, displayed exceedingly low AT III heparin cofactor activity (13% and 16% of the normal, respectively) while values around 50% of the normal were recorded in their parents who had not experienced thrombotic episodes. Since plasma antithrombin III antigen and total progressive antithrombin III activity were within normal limits in all the investigated members of this family it was considered that the two brothers were homozygotes or compound heterozygotes and the parents were heterozygotes for a qualitative-antithrombin III deficiency caused by an abnormality of the heparin binding site.