Motor and sensory conduction studies, F-wave latency, and the latency of evoked potentials from the brachial plexus and cervical spine in response to median nerve stimulation were examined in 17 patients with acute idiopathic polyneuropathy. Short latency evoked potentials provided a higher yield of evidence of abnormalities of proximal conduction than did the F-wave latency. Both methods of studying conduction in the proximal segments of a peripheral nerve may be normal, however, in a small number of patients who have significant pathological lesions.