Five patients with unilateral myoclonus and a clinical diagnosis of corticobasal degeneration (CBD) were studied. All patients showed enhanced long-loop responses in their myoclonic arms without enlarged somatosensory potentials. The cortical relay time of the long-loop responses was studied in three patients, in two of whom it was < 2 ms, even in the nonmyoclonic arm. Myoclonus in CBD is probably related to an enhanced long-loop reflex whose pathway is unlikely to be the same as that in classic cortical reflex myoclonus.