The coagulation and fibrinolytic systems were analyzed in 55 patients with autoimmune diseases. Significant changes, which were found exclusively in those patients with clinically active disease, consisted of thrombocytosis, elevated factor V and factor VIII levels, hyperfibrinogenemia, and evidence of activation of the fibrinolytic system. Three patients with systemic lupus erythematosus had thrombocytopenia. The β1C-globulin was qualitatively low in 11 cases. No correlation between the coagulation or fibrinolytic data and the reduced β1C-globulin could be made. No patient had laboratory evidence compatible with a consumption-coagulopathy. The study suggests that the changes observed were similar to other inflammatory states and were probably nonspecific in nature.