A 15 year-old female presented with anti-acetylcholine receptor antibody-positive myasthenia gravis and electrophysiological signs of sensory peripheral neuropathy. The repetitive stimulation test showed decremental response of 31% in the median nerve. The clinical picture improved with prostigmine, corticosteroids, plasmapheresis and thymectomy. This is the first case report of an Amazon indigenous patient with myasthenia gravis.