The status of pulmonary circulation is of utmost importance to the success of the modified Fontan operation. Pulmonary artery distortion, hypoplasia of the total pulmonary vascular bed, and elevated pulmonary vascular resistance are all risk factors for adverse outcome after modified Fontan operations. In cases of irreparable acquired atresia of one or the other branch pulmonary artery, clinicians are forced to contemplate the possibility of total cavopulmonary connection to one lung. The combined experience of the authors with 12 cases suggests that the likelihood of operative survival following Fontan's operation to one lung is predicted based on the usual hemodynamic parameters: pulmonary artery pressure and flow, ventricular end diastolic pressure, transpulmonary gradient, and pulmonary vascular resistance. In this series, there were no operative mortalities among patients undergoing Fontan's operation to one lung, all of whom meet the usual criteria for hemodynamic acceptability. There may, however, be a higher incidence of protein-losing enteropathy than in Fontan patients with normal pulmonary vascular beds. All possible means of resuscitating the lost elements of the pulmonary vascular bed and re-establishing pulmonary artery continuity should be attempted to minimize pulmonary vascular capacitance of patients undergoing Fontan's operation. It is clear, however, that the presence of only one pulmonary artery does not in and of itself preclude satisfactory outcome.