We report a patient with myoclonic epilepsy who underwent muscle biopsy for suspected mitochondrial disease (myoclonic epilepsy with ragged-red fibers, MERRF). In spite of normal histochemical studies and of the absence of a severe COX deficiency, the molecular analysis showed the common MERRF mutation (A8344G) in the tRNA(Lys), gene on mitochondrial DNA. The case serves to illustrate the importance of pursuing the proposed mitochondrial genetic abnormality, even in patients with normal biopsy findings. (c) 2007 Elsevier Inc. All rights reserved.