OBJECTIVE: To determine the incidence of supratherapeutic international normalized ratio (INR) values in patients initiated on warfarin in the pediatric intensive care unit after the Fontan procedure. DESIGN: Retrospective chart review. SETTING: Pediatric intensive care unit in a freestanding pediatric teaching hospital. PATIENTS: Thirty-two consecutive children with single ventricle physiology with an initial Fontan conversion. All were initiated on warfarin in the postoperative period while in the intensive care unit. OUTCOME MEASURES: Supratherapeutic INR values, interventions in warfarin dosing to address or prevent supratherapeutic INR values, major and minor bleeding episodes, and venous thromboembolic events. RESULTS: Supratherapeutic INR values after warfarin initiation occurred in 12.5% (n = 4) of patients with an additional 6.3% (n = 2) requiring dose adjustments due to a rapidly increasing INR. Patients with a supratherapeutic INR were started on warfarin earlier compared with other patients (median postoperative day 2 vs. 5, P = .037, respectively). There were no reported thrombi or thromboembolic events and the only clinically significant bleeding event occurred while the INR value was subtherapeutic. CONCLUSIONS: Patients undergoing the Fontan procedure may be more sensitive to warfarin dosing when initiated closer to the surgical procedure date. Despite this observation, warfarin appears to be a safe medication for use within the early postoperative period after the Fontan procedure when response to therapy is monitored closely. Clinicians should use caution when initiating warfarin before postoperative day 3 in patients undergoing an initial Fontan operation.