Is it possible to advise either for or against prophylactic anticoagulation in patients following Fontan surgery? Certainly, the evidence suggests that thromboembolism is a major risk factor and it may therefore be considered reasonable to anticoagulate all patients. However, key information is missing regarding the safety and benefit from aspirin and warfarin treatment, as well as target international normalised ratio(INR) levels and timing of therapy. Consequently, if we are to subscribe to an evidence based approach, a blanket recommendation cannot be given. Nevertheless, if a high risk group is to be identified, patients with progressive right atrial dilatation and/or a history of atrial arrhythmia would appear, albeit anecdotally, to present the strongest case for prophylactic anticoagulation. Clearly, prospective controlled studies are required before definitive recommendations can be made. Is it possible to advise either for or against prophylactic anticoagulation in patients following Fontan surgery? Certainly, the evidence suggests that thromboembolism is a major risk factor and it may therefore be be considered reasonable to anticoagulate all patients. However, key information is missing regarding the safety and benefit from aspirin and warfarin treatment, as well as target international normalized ratio (INR) levels and timing of therapy. Nevertheless, if we are to subscribe to an evidence based approach, a blanket recommendation can not be given. Nevertheless, if a high risk group is to be identified, patients with progressive right atrial dilatation and / or a history of atrial arrhythmia would appear, albeit anecdotally, to present the strongest case for prophylactic anticoagulation. Clearly, prospective controlled studies are required before definitive recommendations can be made.