In order to study the usefulness of incorporating a subpulmonary ventricular chamber (SPVC) in the Fontan circulation, we compared the hemodynamics of 18 patients who underwent an atriopulmonary connection with those of 17 patients with an atrioventricular connection. Early postoperatively, regardless of whether an SPVC was incorporated or not, the mean pulmonary artery pressure was not higher than the mean right atrial pressure in any of our patients. The stroke work of the right heart was at best zero, and pressure tracings showed an a wave in the pulmonary artery (right atrium-dependent circulation). If an additional gradient was interposed between the right atrium and pulmonary artery, the effective stroke work of the right heart might be negative and the work curve have a negative slope. This can occur irrespective of the use of an SPVC. In the long term, if it grows sufficiently, an SPVC can sometimes provide a right ventricle-dependent pulmonary circulation with a more normal hemodynamic pattern (mean right atrial pressure less than mean pulmonary artery pressure). We concluded that an SPVC should be used in Fontan's circulation, provided that an obstruction-free atrioventricular connection can be established.