A 28-year-old male with a strong family history of thromboembolic disease sustained three arterial thromboembolic occlusions during a 7 mo. period. The patient had antithrombin III (AT III) levels of 19.2 mg/dl (N=17-30 mg/dl) by RID and an immediate (heparin activated) antithrombin level of 77% (N=88-120%). Crossed IEP showed normal electrophoretic mobility of the patient's AT III. Of 30 family members tested, 10 demonstrated decreased AT III by both immunologic and functional assays. Two children of the propositus and three children of the sister of the propositus were tested and none were found to be abnormal. Because of the unusual presence of arterial thrombotic events in this family, platelet function studies were performed on the propositus and on two AT III deficient family members. A 19-year-old brother of the propositus, with no history of thrombotic events but with 13 mg/dl AT III and 83% activity had normal platelet aggregation studies and demonstrated no evidence of hyperaggregability with suboptimal concentrations of aggregating agents. He did, however, demonstrate a slight increase in platelet adhesion in a collagen adhesion assay. The second family member, a 65 year-old aunt of the propositus with 13.7 mg/dl AT III and 75% activity was on Coumadin at the time of evaluation of platelet function. Platelet aggregation and adhesion studies were normal in this individual. The propositus was also tested while on Coumadin and showed no abnormality of platelet aggregation, no hyperaggregability and no increase in platelet adhesion.In view of our experience, we recommend screening for AT III deficiency in patients with unexplained recurrent arterial thromboembolism as well as those with venous thromboembolism, especially if the family history is suggestive.