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Muscular coenzyme Q10 deficiency can show a spectrum of clinical manifestations: a severe infantile encephalomyopathy with renal involvement, a cerebellar variant, a Leigh-like syndrome and an isolated myopathy with lipid storage. Primary defects of CoQ10 biosynthesis have so far been described in patients with the infantile nephrotic form. It was recently suggested that the myopathic form of CoQ deficiency was secondary to an ETF-dehydrogenase deficiency (Gempel et al., 2007). Methods. The mitochondrial respiratory chain and fatty acid oxidation were investigated in a 45-year-old patient with severe lipid storage myopathy and lactic acidosis using biochemical and molecular genetic methods. Results. The activities of the individual respiratory chain enzymes were normal, but the combined activities of Cx I-III and Cx II-III were reduced in muscle. Coenzyme Q10 levels were severely reduced in cultured fibroblasts. Plasma acylcarnitine analysis indicated a multiple acyl-CoA dehydrogenase defect, but complete sequencing of ETF-A, ETF-B and ETF-DH did not reveal any pathogenic mutations. Conclusions. We present a patient with severe coenzyme Q10 deficiency who fulfils all criteria for the myopathic form of the disease and in whom ETF-dehydrogenase mutations and respiratory chain disorders have been excluded. Our findings indicate that coenzyme Q10 deficiency can lead to a secondary impairment of mitochondrial fatty acid oxidation.