The pattern of pulmonary blood flow was studied in three patients after a total cavopulmonary shunt procedure. Doppler studies showed a phasic pattern of flow which varied with the respiratory cycle. Pulmonary blood flow was increased with normal inspiration, and was much augmented by the Mueller manoeuvre. This suggests that flow was occurring when a negative intrathoracic pressure was generated. During a brief Valsalva manoeuvre blood flowed away from the lungs. With a sustained Valsalva manoeuvre there was no spontaneous forward flow; instead there was low velocity pulsatile pulmonary blood flow that coincided with ventricular systole. Left ventricular cavity dimensions decreased, reflecting a considerably reduced pulmonary blood flow. Pulmonary blood flow after the total cavopulmonary shunt operation is critically dependent on changes in intrathoracic pressure. This has important implications in terms of the immediate postoperative management of these patients.