A patient with chronic Leigh's necrotizing encephalomyelopathy also had cardiomyopathy and a prominent “mitochondria1 myopathy.” It is speculated that (1) muscle often may be involved in Leigh's disease and might serve as an important tissue for biochemical investigation, (2) certain “neurodegenerative disorders” known in the literature by other terms actually may be Leigh's disease, and (3) similar bizzarre mitochondria may be of more than secondary importance in other disease entities.