Disturbances in surround inhibition could account for various movement disorders. Here we test the functional operation of surround inhibition in focal hand dystonia. Transcranial magnetic stimulation was set to be triggered by self-initiated voluntary flexion of the index finger. During this movement, motor-evoked potential amplitudes from the little finger muscle were significantly suppressed in healthy subjects but enhanced in dystonia patients. This result supports the idea that disturbed surround inhibition is a principal pathophysiological mechanism of dystonia.