Objective: To evaluate the effect of the AT III concentrates upon the clinical evolution and hemostatic parameters. Design: Prospective, open, randomized trial. Patients and participants: Septic and multiple trauma patients admitted to our Intensive Care Unit. Setting: Levels of AT III below 70% were used as criteria to choose 36 patients, 20 of whom received treatment with ATIII and 16 did not. Interventions: ATIII concentrates were administered at an initial dose of 60 U/kg followed by 10 U/kg every six hours. Results: The administration of AT III neither contributes to alterations in haemostasis, nor the clinical evolution (evaluated according to Apache II score). Conclusions: The results suggest that the administration of ATIII concentrates to critical patients with acquired low levels, but without manifest DIC, may not be justified; although further studies on a larger population are required to establish definite conclusions.