We reviewed the clinical records of 185 patients who were possible candidates for Fontan operation. We did this to clarify whether all patients with suitable anomalies ultimately underwent a Fontan operation. Of the 152 patients with decreased pulmonary blood flow, 38 (26%) did not fulfill the criteria for Fontan operation, and 17 of the 33 patients (52%) with increased pulmonary blood flow (P < 0.01) were excluded as candidates. Of 48 non-candidate survivors, 19 had high pulmonary artery (PA) pressure or resistance and small PA (which we term "poor PA"), 17 had pulmonary hypertension, 6 had a markedly distorted PA, and 6 had severe ventricular dysfunction. A significant proportion of possible candidates with a suitable anomaly ultimately did not undergo a Fontan operation, because of "poor PA", a congenital condition that precluded Fontan operation.