Using surface electrodes over the lower chest wall, we have measured the phrenic nerve conduction time, i.e. the time interval between the stimulation of the phrenic nerve in the neck and the onset of the diaphragmatic muscle action potential, in 31 control subjects (bilaterally in 19 of them) and in patients in whom the phrenic nerve was at risk from intrathoracic disease. The data obtained from the patients showed that 1 degrees this technique, unlike fluoroscopic examination of the diaphragm, provides quantitative information on phrenic nerve function; in particular, it may detect partial impairment of the nerve by a neoplastic process; 2 degrees it allows an accurate assessment of phrenic nerve function even when lung collapse, pleural effusion, or pleural adhesions make it difficult to visualize the movement of the diaphragm; and 3 degrees it avoids the falsepositives seen on fluoroscopy. We suggest that measurement of phrenic nerve conduction time should be preferred to diaphragmatic fluoroscopic screening in the evaluation of phrenic nerve function (Acta neurol, belg, 1982, 82, 91-98).