A 19-month-old girl presented with progressive myoclonic jerking of both proximal lower extremities. On her brain magnetic resonance imaging (MRI), the authors found an ill-defined mass involving cerebellar vermis and the right middle cerebellar peduncle. ( 11)C-methionine positron emission tomography (PET) showed no abnormalities, but (18)F-fluorodeoxyglucose ((18)F-FDG) PET revealed a well-defined hypermetabolic focus. Depth electrodes were inserted deep into the mass, which recorded focal slow waves associated with the clinical myoclonus. Following the removal of the tumor, the myoclonus was completely resolved with no neurological deficit. Here, the authors present a case showing progressive myoclonus associated with a cerebellar ganglioglioma with the electrophysiological data, which provides strong supportive evidence that the cerebellum can be a myoclonus generator.