A patient with intention and action myoclonus, epilepsy, ataxia, and mental deterioration in association with ragged-red fibers in striated muscle is described. This patient demonstrated a unique form of erratic myoclonus with continuous EEG spike activity during eye closure. Both the myoclonus and the EEG spiking disappeared on opening the eyes. A defect in the activity of complex I in the respiratory chain was demonstrated.