The authors examined a 35-year-old man who had multifocal osteonecrosis on the right and left femoral heads, left humeral head, and right scaphoid. The patient previously had a diagnosis of Type I congenital antithrombin III deficiency but had no other risk factors for osteonecrosis. This patient was thought to have systemic thromboembolism because of the increased blood coagulability resulting from antithrombin III deficiency. Thrombophilia (increased likelihood of thrombosis) and hypofibrinolysis (reduced ability to lyse thrombi) have been implicated as causative factors for osteonecrosis. This case shows a possible relationship between thrombophilic antithrombin III deficiency and multifocal osteonecrosis.