The plasma levels of AT III were studied in 71 patients operated with total hip replacement. AT III was determined with amidolytic assay using the chromogenic substrate Coatest S-2238 (KabiDiagnostica, Sweden) as well as electroimmunochemically. Samples were drawn before the operation and 4-5 times the first week postoperatively. The results were adjusted to the hematocrit (Hcr) for each sample.The patients were randomly allocated to thromboprophylactic prevention with either Macrodex (Pharmacia, Sweden) or heparin with dihydroergotamine (Sandoz, Switzerland). Phlebography of the operated leg and perfusion/ventilation lung scanning was performed on the 10-14th postoperative day.Results. No difference in the AT III levels were seen in patients who developed postoperatively deep venous thrombosis and/or pulmonary embolism (DVT/PE) as compared to those who did not develop DVT/PE.No decrease of AT III was found postoperatively, when adjusted to HcrThe frequency of DVT/PE did not differ significantly between the two types of thromboembolic prophylaxis.Conclusion. Pre-operative determination of AT III, whether biologically or immunochemically, did not seem to be successful as screening method of patients to develop DVT/PE.