In order to determine what kind of voluntary movement induces action myoclonus, we gave two siblings with sialidosis two kinds of tasks. When the patients were asked to move their index fingers following a smoothly moving target, action myoclonus became prominent. In contrast, when they were asked to perform the same movements with their eyes closed, they could move their index fingers very smoothly. This shows that action myoclonus was induced by visually guided movement, but not by self-paced movement. Our observations might reflect a disorder of the cerebellum, which controls visually guided movement.