We observed the changes of parameters of coagulation and fibrinolytic system in order to understand the clinical implication of these variations in type I diabetic patients. Subjects consisted of 22 patients with type I diabetes mellitus and 25 healthy controls. Compared with the control, activated partial thromboplastin time, prothrombin time were shortened in the patients. The diabetic subjects also displayed higher levels of D-dimer, serum fibrin degradation products, median concentrations of fibrinogen (3. 99 vs 2. 96 g/L,P<0. 01) and von Willebrand factor (149 % vs 87 %,P<0.01). Levels of antithrombin activity or antigen were not different from control values. Simple linear regression analysis revealed a negative correlation between antithrombin activity and fast blood glucose. Diabetic patients with vascular complications had significantly higher levels of fibrinogen and D-dimer than those without diabetic angiopathy. Our data demonstrated that patients with type I diabetes mellitus had a hypercoagulable state. We believed the activation of coagulation might contribute to the vascular complications in diabetics.