The authors compare the clinical, neurophysiological and evolutive features of progressive myoclonus epilepsy (PME) associated with mitochondrial encephalomyopathy with ragged-red fibers (MERRF), based on 49 cases from the literature, and the two well-described types of degenerative PME: Baltic myoclonus (BM), of which over 100 cases have been reported from Finland, and Mediterranean myoclonus (MM), based on a personal series of 43 patients. Degenerative PMEs are age-dependent, recessively inherited conditions with homogeneous clinical signs and course; there are no major clinical symptoms beside the cardinal symptoms: generalized epileptic seizures, predominantly action myoclonus and cerebellar dysfunction; mental deterioration when present, is slight and progresses very slowly; associated neurological symptoms are uncommon and limited to mild spino-cerebellar involvement. In MERRF, the transmission is maternal, the age of onset is variable, the evolution is not stereotyped and associated symptoms are many (deafness, muscle weakness, optic atrophy, short stature, sensory disturbances, spasticity, clinical or neurophysiological signs of peripheral neuropathy, absence of motor reflexes); muscle biopsy generally shows ragged-red fibers. The differential diagnosis between these conditions is usually easy, although pathological examination (i.e. muscle biopsy) should be performed.