Twenty-four neonates at 25-34 weeks' gestation with a weight range of 570-1530 g underwent ligation of patent ductus arteriosus (PDA). The infants had mild to severe respiratory distress syndrome at birth and later developed signs of heart failure as a result of left-to-right shunting through a PDA. Surgical closure of the PDA was performed within 2-31 days after birth. In the period before operation the heart rate was monitored constantly and the arterial blood-gases were assessed frequently. The trachea was intubated and respiration was controlled with a ventilator. Surgery was performed under controlled ventilation and no anaesthesia was used. Care was taken not to overventilate the lungs. Nine infants died. Death was associated with higher peak inspiratory ventilator pressures at the time of operation and with complications occurring during or after the operation. The most common complication was tension pneumomediastinum which appears to be related to excessive ventilator pressures during surgery.