An 18-year-old woman presented with dissociated nystagmus that was first attributed to multiple sclerosis. When she attempted to maintain 30-degree horizontal gaze deviation in either direction, there was progressive weakness of the adducting eye and increasing amplitude of nystagmus in the abducting eye. The amplitude and maximum velocity of repetitive one-per-second 30-degree saccades progressively decreased over a 3-minute period. Intravenous edrophonium completely abolished the dissociated nystagmus and adducting muscle weakness, and returned the saccade maximum velocity to normal. Her subsequent response to pyridostigmine and thymectomy confirmed the diagnosis of myasthenia gravis.