In 15 patients who underwent open exploration of the brachial plexus, the somatosensory evoked potentials and nerve action potentials recorded at the time of operation were useful as guides to the most appropriate surgical procedure, and also in predicting the outcome in certain lesions. In three patients the apparent normality of the upper trunk of the plexus was concealing a more proximal lesion which was irrecoverable. The presence of a somatosensory evoked potential showed functional continuity in three patients in whom the C7 root was clinically involved and who recovered after operation. In five patients proximal stumps of ruptured C5 roots showed functional central continuity; this indicated their suitability for grafting. These patients recovered except one who suffered from co-existing disease. The electrophysiological studies also confirmed the clinical diagnosis of avulsion of the C8 and T1 roots and therefore prevented unnecessary dissection.