BACKGROUND-Mitochondriopathy has been rarely reported to imitate motor neuron disease.REVIEW SUMMARY-A 57-year-old, 157-cm-tall woman with clinical and electrophysiological features of motor neuron disease since 1993 is reported. She also had increased liver function parameters, hypothyroidism, and sinus tachycardia. Because her mother and sister had both died from assumed amyotrophic lateral sclerosis (ALS), familial ALS was diagnosed. On reevaluation, screening for superoxide-dismutase gene mutations was negative, but lactate stress testing was abnormal and muscle biopsy revealed patchy COX deficiency and abnormal mitochondria. Analysis of the muscle mtDNA revealed substitutions in the isoleucine tRNA, in the ATPase-6, and in the cytochrome-b gene, respectively. Based on these data, the diagnosis of ALS was changed to mitochondriopathy.CONCLUSIONS-Mitochondriopathy may mimic ALS, phenotypically and electrophysiologically. In patients with an ALS phenotype, slow progression, and multisystem involvement, mitochondriopathy should be considered a diagnostic possibility.