To determine its potential impact on perioperative surgical management intraoperative ultrasound examination (cross sectional imaging, colour flow mapping, pulsed and continuous wave Doppler) was used in 16 consecutive patients undergoing a Fontan-type procedure. Epicardial cross sectional imaging before bypass defined the precise intracardiac morphology in 15 of 16 patients. The preoperative morphological diagnosis was refined in four patients (25%), and this influenced surgical management in two (12%). Epicardial studies after bypass identified seven residual haemodynamic lesions in five patients (three residual intercardiac shunts, one ventricular outflow obstruction, one pulmonary artery obstruction, two mitral valve regurgitation), and led to immediate revision during a second period of bypass in three (18%). In one patient who required early reoperation residual shunting was not detected after bypass by either colour flow mapping or a contrast study. Final intraoperative studies showed a good surgical result in 14 patients (87%). Flow characteristics and flow velocities within the Fontan circulation could be assessed immediately after the patient came off cardiopulmonary bypass by means of combined pulsed wave Doppler and colour flow mapping in 14 of the 16 patients. Cross sectional studies of the left heart after bypass showed no change in ventricular function and allowed monitoring of volume replacement and ventricular filling. Intraoperative ultrasound was a valuable monitoring technique in patients undergoing a Fontan-type procedure. It refined preoperative diagnosis, monitored ventricular function, and identified or excluded residual haemodynamic lesions in most patients.