We studied the contraction of skeletal muscle upon direct percussion in 119 patients presenting with palsies related to focal peripheral nerve disorders. We compared the response of the paralyzed muscles with the response obtained on the contralateral normal side. The cause of the palsy was subsequently assessed by electrophysiological testing which disclosed: conduction block (CB), axonal, or mixed lesions (when both CB and denervation existed in similar proportions). Most of the time, the muscle response to percussion was increased in patients with CB (37/47), decreased in axonal lesions (44/53), and symmetric in mixed lesions (12/19). We conclude that distinction between CB and denervation is often possible on the basis of this simple bedside test.