A 34-year-old woman with long-standing multiple sclerosis had a 2-year history and physical signs of myasthenia gravis. The edrophonium chloride (Tensilon) test was positive. Repetitive stimulation of the ulnar nerve at 3 Hz did not show evidence of myasthenic response; however, a single-fiber electromyography demonstrated evidence of neuromuscular block as seen in myasthenia gravis, which was reversed to normal after intravenously administered edrophonium. The patient improved on anticholinesterase medication. It is suggested that patients with multiple sclerosis who have unusual features such as in the patient reported here should be investigated for the presence of myasthenia gravis to ensure proper treatment.