OBJECTIVE--To document whether the act of respiration influences pulmonary blood flow in patients after the Fontan operation. DESIGN--Prospective study in which patients acted as their own controls. SETTING--Supraregional paediatric cardiology centre. PATIENTS--Sixteen patients who had undergone atriopulmonary anastomosis (Fontan operation) for the treatment of congenital heart disease. INTERVENTION--Doppler assessment of pulmonary artery flow patterns with a simultaneous respirometer, electrocardiogram and phonocardiogram. RESULTS--Total (mean (1SD) ) forward pulmonary flow was 63.6 (35)% higher during inspiratory cardiac cycles than during expiratory ones. CONCLUSIONS--The act of breathing provides an additional energy supply to pulmonary blood flow in patients after the Fontan operation.