Immediate and long-term results of the Fontan procedure depend on the value of the pulmonary vascular resistance. This parameter cannot always be quantified accurately during preoperative evaluation. Cavo-pulmonary bypass using an extra corporeal circulation allowed simulation a Fontan procedure in 5 patients, and prediction of the pressure changes in the pulmonary artery and the systemic atrium according to the pulmonary to systemic resistances ratio. This technique could be valuable in border line cases for deciding on the optimal therapeutic solution.