Four autopsied cases of myoclonus, ataxia, and epilepsy from 2 separate pedigrees are described. An identical pattern of focal brainstem lesions was found in all the cases with selective and symmetrical degeneration of the dentate and second order somatosensory nuclei. The combined clinical and pathological features did not appear to match any familial disorder previously described as causing progressive myoclonus epilepsy. Myoclonus epilepsy with ragged red fibres was excluded on the grounds of paternal inheritance and negative muscle biopsy findings, but the more acute lesions seen in 1 case are reminiscent of those found in Leigh's syndrome, and suggest that some other form of inherited defect of oxidative metabolism may be involved.