Some studies have suggested that thrombotic and fibrinolytic disorders may be etiologic causes of osteonecrosis of the femoral head. A case-control study was done to determine whether these disorders are associated with osteonecrosis of the femoral head in East Asian patients with nontraumatic osteonecrosis of the femoral head. Twenty-four consecutive patients who had been diagnosed as having nontraumatic osteonecrosis of the femoral head were matched with 24 control subjects for gender, age (1-year range), and the time of presentation (1-year range). Thrombotic factors including protein C activity, protein S activity, antithrombin III, anticardiolipin antibody immunoglobulins G and M, and lupus antibody were investigated. Fibrinolytic factors including tissue-plasminogen activator, plasminogen activator inhibitor-1, tissue-plasminogen activator and plasminogen activator inhibitor-1 ratio, lipoprotein (a), and plasminogen also were investigated. There were no significant differences in the levels of thrombotic and fibrinolytic factors. In eight patients with idiopathic osteonecrosis, anticardiolipin antibody immunoglobulin G, an antiphospholipid antibody which is associated with thrombotic phenomena, was lower than that in respective control subjects. These data do not confirm an etiologic role for thrombotic and fibrinolytic disorders in East Asian patients with nontraumatic osteonecrosis of the femoral head.