We used phosphorus magnetic resonance spectroscopy (P-31-MRS) to study in vivo brain and muscle bioenergetics in a male patient with Leber's hereditary optic neuropathy (LHON) and mtDNA mutation at 11778 bp who developed spastic paraparesis with white matter lesions on brain MR imaging. The study was performed before and during treatment with idebenone (135 mg t.i.d.) and after withdrawal. Clinical amelioration and worsening were associated with parallel changes in brain and skeletal muscle bioenergetics following the administration or withdrawal of idebenone. Reversal of paraparesis by idebenone was paralleled by normalization of P-31-MRS, serum lactate and central motor conduction. Extra-ocular neurological dysfunction in LHON may be amenable to treatment by appropriate quinones. (C) 1997 Elsevier Science B.V.