OBJECTIVE--To assess whether regional abnormalities of ventricular function are present in patients after the Fontan operation and to explore the implications of any such abnormalities for ventricular filling. DESIGN AND PATIENTS--Prospective study in which 25 patients after the Fontan operation were compared with 25 healthy controls and 12 patients with a univentricular atrioventricular connection, before the Fontan operation. INTERVENTIONS--Doppler echocardiography, with simultaneous electrocardiogram, phonocardiogram, and respirometer. RESULTS--Isovolumic relaxation time was significantly longer in patients after the Fontan operation than in normal children (p less than 0.001) or the preoperative patients (p = 0.001). Systolic intraventricular flow was detected in 60% of patients after the Fontan operation and in 42% of preoperative patients. After the Fontan operation 80% of patients showed intraventricular flow during isovolumic relaxation compared with 8% of normal children and none of the preoperative patients. CONCLUSIONS--Incoordinate ventricular relaxation is common after the Fontan operation. This may have important implications for ventricular diastolic filling, pulmonary blood flow, and cardiac output in these patients.