Patients with diabetes mellitus also frequently have atherosclerotic diseases such as myocardial infarction and/or stroke which exert a great influence on prognosis. We measured levels of thrombin-antithrombin III complex (TAT), crosslinked fibrin degradation products (XDP), tissuetype plasminogen activator (t-PA), and elastase-α1 proteinase inhibitor complex (E-α1 PI) in plasma in 20 diabetics. Levels of TAT, which is considered to be a sensitive marker of intervascular activation of prothrombin, and E-α1 PI, which is thought to reflect the amounts of elastase derived from polymorphonuclear neutrophils (PMN) were found to increase significantly in diabetics compared to controls. Levels of t-PA, which converts plasminogen to plasmin, tended to decrease and concentrations of XDP were within normal limits in diabetics. The levels of E-α1 PI in plasma of diabetics with high levels of TAT (5.0<) was lower than those with low levels of TAT, although this was not statistically significant. From these results, we conclude that hypercoagulability and impaired fibrinolysis are present in diabetics which enhance thrombotic tendency. The fact that concentrations of E-α1 PI in plasma did not increase when levels of TAT increased indicates that host defense to development of atherosclerosis by PMN-elastase might be inhibited in diabetics with hypercoaglability.