A 23-year-old man presented with a history characterized by a myoclonic syndrome developing over a period of seven years. Predominant symptoms were intention and activity myoclonus, generalized epileptic seizures occurring infrequently from the age of 20, a slowly progressive cerebellar syndrome first apparent at 19 years, and the sudden onset of loss of visual acuity at 19, which then partially regressed; optic atrophy and clinical and campimetric signs were suggestive of Leber's disease. Intellectual ability was not affected. E.E.G. records showed generalized spike-waves with photosensitivity, progressive reduction in basal rhythm, and sleep organization disturbances with focal abnormalities. Obvious clinical signs of muscle disease were lacking but muscle biopsy confirmed the presence of a mitochondrial myopathy (ragged-red fibers). An indefinite history of familial neurological disease was obtained. Diagnosis was established as myoclonic cerebellar dyssynergy with spastic hereditary ataxia and Leber's disease. Their association with a mitochondrial myopathy has been previously reported by Tsairis et al, Fukuhara et al, Fitzimons et al (familial case), and Niedermeyer et al (sporadic case). In spite of the non-specific nature of associated mitochondrial abnormalities, all these cases would appear to correspond to a single nosological entity.