Several studies have indicated that a number of different mitochondrial DNA (mtDNA) mutations may be responsible for human pathologies. Sensorineural Hearing Loss (SNHL) may be associated with known syndromes (syndromal SNHL) or represent the only manifestation of mitochondrial damage (non-syndromal hearing loss). Moreover, mtDNA alterations may be responsible for aminoglycoside-induced deafness. We describe a patient harbouring a single sporadic mtDNA deletion, who presented with sudden adult-onset bilateral, although non-simultaneous SNHL, that was partially responsive to corticosteroids. Increased values of rest, and exercise, blood lactic acid were decisive for diagnosis, prompting muscle biopsy that revealed the mtDNA deletion. The case underscores the importance of investigating a mitochondrial disease in cases of SNHL of unknown origin and points out the importance of an increased blood level of lactic acid as a screening test.