A 36-year-old male developed an acute right-sided weakness due to left-sided pontine infarction. Two months later, he first noticed sudden right elbow flexion in response to a loud unexpected noise. Detailed electrophysiological assessment was performed. A large, short-latency (median 39 msec), synchronous electromyographic discharge occurred in the right biceps brachii electrodes following a 50-msec, 120-dB 1-kHz tone burst, with habituation only with very short (30-second) interstimulus intervals. Less synchronous activity at longer latencies was present both in a number of right-sided arm muscles at rest and on the clinically unaffected side during a tonic voluntary contraction. We discuss possible underlying mechanisms and our reasons for considering this a focally enhanced startle response. Our report broadens the range of expression of acquired startle disorders. © 2001 Movement Disorder Society.