Over a 2-year period (1987-1988), 31 children (3% of 960 operations) were found to have a paralysed diaphragm following cardiac surgery. The median age was 5 months with 65% less than 1 year of age. In the 31 patients, 38 paralysed diaphragms were identified. The phrenic nerve injury was on the right in 8 cases, the left in 16, bilateral in 7 and 40% were related to modified Blalock Taussig shunts. The time to extubation was analysed for each patient in two separate groups: Group A: (greater than 1 year): 11 children of whom 5 were extubated without difficulty, 4 had non-respiratory reasons for prolonged ventilation and only 2 were plicated--both were extubated within 4 days. Group B: (less than 1 year): 20 infants of whom 16 were plicated and 4 were not. Three of the children who were plicated died from cardiac causes. Of the 13 survivors, there were 3 who had other reasons for prolonged ventilation. Thus 10 infants required prolonged ventilation (mean 11 days) because of respiratory difficulties. All underwent plication and were extubated at a mean of 2.4 days postoperatively. The 4 who were not plicated were extubated at a mean of 11 days postoperatively. In infants in whom there is no cardiac cause for failure to wean from ventilation, diaphragmatic paralysis should be suspected and plication performed if not extubated 2 weeks after operation.