Identification of a family affected by antithrombin III-heparin cofactor (AT-III) deficiency was made after diagnosis of the index case, a 15-year-old boy who suffered cerebral thrombosis. The proband had a two-year history of recurrent thrombosis involving the lower extremities. His mother and sister were also affected. Studies showed a decreased biological activity (AT-IIIc) and antigen (AT-IIIag) by the Laurell technique in the proband (AT-IIIc = 0.32, AT-IIIag = 46%), his sister (AT-IIIc = 0.29, AT-IIIag = 47%), and his mother (AT-IIIc = 0.41, AT-IIIag = 56%). Crossed immunoelectrophoresis (CIE) of the affected individuals' plasma in agarose-containing heparin demonstrated a normal pattern of migration. Treatment with warfarin sodium (Coumadin) resulted in an increase in activity in two of three affected family members, and in antigen in all three. Anticoagulant therapy did not affect the pattern of AT-III on CIE. This family represents a quantitative deficiency in antithrombin III. A review of the reported cases of antithrombin III deficiency indicates that individuals with this disorder may have thromboembolic disease in childhood.