Myasthenia gravis and mitochondrial myopathy may present with similar clinical symptoms as inconstant palpebral ptosis, ophthalmoparesis, and muscle weakness. A few case initially diagnosed as myasthenia gravis by a positive decremental response on EMG and successful anticholinesterase therapy revealed to be affected by mitochondrial disease. We report a new case initially found to be affected by myasthenia gravis in whom muscle biopsy, performed because of symptom worsening, disclosed a mitochondrial myopathy. It is not clear if the association of mitochondrial myopathy and myasthenia gravis is coincidental or if there is a pathogenic link between the two pathologies. We suggest that muscle biopsy should be performed in cases with atypical myasthenia gravis signs.