Mitochondrial disorder is an inborn error of metabolism affecting the cellularrespiratory chain. Defective energy production leads to a wide variety of clinicalmanifestations (ataxia, epilepsy, dementia, myopathy, polyneuropathy, retinal pigmentanomalies, and cardiomyopathy with conduction anomalies). Hearing loss is a regularfeature and is often the first clinical symptom. Audiologic data from 26 members of afamily in three generations is presented. One of these patients was examined for thebiochemical error. Respiratory study of muscle biopsy revealed a mild defect in theNADH-ubiquinone oxidoreductase step of the oxidative phosphorylation (complex I).The content of cytochrome aa3(complex IV) was also reduced. Adult onset sensorineuralhearing loss starting in the high frequency region progresses with a fairly constant speedin this family. A cochlear type of hearing loss is found in the less pronounced cases.Advanced cases present features of retrocochlear affection with decreasing speechrecognition, elevated acoustic reflex thresholds, and increased ABR latency with de-rangement of potentials. Caloric sensitivity was unaffected.