Six (1.2%) of 501 patients sustained phrenic nerve injury during operation for congenital heart disease at our institutions between 1992 and 1998. The diagnosis was confirmed by percutaneous stimulation of the phrenic nerve. All but 1 patient were less than 9 months old, and the average weight was 3.6kg. All 6 patients underwent diaphragmatic plication and were extubated by 7 days after operation. Percutaneous stimulation of the phrenic nerve allowed direct assessment of phrenic nerve function which was difficult to detect by clinical and radiological evidence. This method can be non-invasively used at the bedside to facilitate early and accurate diagnosis of phrenic nerve palsy.