Phrenic nerve damage is widely recognized after cardiac operations and is associated with an increased morbidity and mortality. Retrospective studies in children have estimated a prevalence of phrenic damage of between 0.5% and 2.2%, but all these studies have limited their investigations to patients who have clinical problems postoperatively, and they have all used clinical or radiologic methods to diagnose the damage. These methods are indirect and hard to evaluate, making the accurate early diagnosis of phrenic nerve damage very difficult. Measurement of phrenic nerve conduction time (phrenic latency) allows direct evaluation of phrenic function and integrity, making it potentially much more specific than indirect methods. Using a simplified method of direct phrenic nerve stimulation, we have developed a method of measuring phrenic latency at the bedside in infants and children. A 1 Hz electrical stimulus is applied over the phrenic nerve in the neck and a diaphragmatic electromyogram from the seventh and eighth intercostal spaces is displayed on a storage oscilloscope. Phrenic latency can be measured directly from the screen. Using these methods we have now studied 37 children (aged 2 days to 15 years) before and after cardiac operations. Mean phrenic latency was 5.4 +/- 1.0 msec on the right and 5.4 +/- 1.0 msec on the left. Prolongation of phrenic latency by more than 2 msec was found in seven of 66 postoperative measurements (10.6%). These patients had a substantially worse postoperative course. We believe this technique to be an important contribution to the diagnosis of postoperative phrenic nerve damage, and one that can help in clinical practice to elucidate an important cause of postoperative morbidity and mortality.