Antithrombin III (ATIII) is used during extracorporeal membrane oxygenation (ECMO) based on physiologic rationale and studies during cardiopulmonary bypass. In February 2008, our institution began using ATIII as replacement for low ATIII activity (<70%) in patients supported with ECMO. We hypothesized that ATIII supplementation would reduce heparin infusion rates, increase unfractionated heparin anti-Xa levels, and prolong ECMO circuit life. Data from 40 consecutive patients (45 deployments) requiring ECMO support for >72 hours with venoarterial ECMO from January 1, 2007, through December 31, 2008, were collected. Antithrombin III concentrate was administered for ATIII activity <70% at the discretion of the attending physician. The primary outcome was whether the heparin infusion rate was reduced by 10% or more as a result of ATIII administration. No difference in heparin infusion rate (p = 0.245) as a result of ATIII administration was observed. Anti-Xa levels were lower before ATIII administration (p< 0.001) and were increased after ATIII administration (p < 0.001). There was an increased frequency of circuit failure in ATIII treatment group compared with nontreatment group (p = 0.018). Neither heparin responsiveness nor circuit life was enhanced by daily ATIII supplementation for activity <70%. Future studies are warranted to evaluate the effectiveness of antithrombin replacement. Copyright © 2013 by the American Society for Artificial Internal Organs.