The most common mitochondrial DNA (mtDNA) mutations giving rise to Leigh syndrome reside in the MTATP6 gene. We report a rare mutation, m. 9185 T>C that gives rise to a progressive, but episodic pattern of neurological impairment with partial recovery. Disease progression corresponded to febrile viral illness and nuclear magnetic resonance imaging (MRI) changes. The patient displayed nearly 100% homoplasmy, while his asymptomatic mother was 30%. Phenotypically, exacerbations of muscle weakness with endurance intolerance, dysarthric speech, ataxia, and eyelid ptosis accompanied febrile viral illness. This case demonstrates an episodic pattern of febrile illness-induced disease exacerbation with corresponding MRI changes. © 2010 Mitochondria Research Society.