Fifty infants and 50 children less than 15 years undergoing palliative or corrective cardiac surgery in the Brompton Hospital between March and October 1988 had direct percutaneous stimulation of the phrenic nerve before and after operation. Ten patients, six under 1 year of age and four over, developed unilateral phrenic nerve injury. In those aged less than 1 year recovery after operation was prolonged because their diaphragmatic palsy made it difficult to wean them from the ventilator. Older children had symptoms but their rate of recovery did not seem to be affected by the phrenic nerve injury. Phrenic nerve damage was no more frequent after a lateral thoracotomy than after a median sternotomy. There was no significant association with the type of operation performed, the experience of the surgeon, the use of bypass or topical ice, the duration of bypass, circulatory arrest or aortic cross clamping, or the age of the patient at the time of operation. In patients who had cardiopulmonary bypass the risk of injury was significantly higher in those who had undergone previous operation. The 10% frequency of phrenic nerve injury determined in this prospective study was higher than that seen in earlier retrospective reports. Direct percutaneous stimulation of the phrenic nerve can be used at the bedside in infants and children to facilitate early and accurate diagnosis of phrenic nerve palsy, and the results may influence early management.