A 61-year-old woman in excellent health and taking no medications presented with progressive dyspnea on exertion. She did not exhibit diplopia, dysphagia, dysarthria or muscle weakness. Her condition deteriorated, and respiratory distress developed. The patient was intubated and placed on mechanical ventilation. A computed tomogram of her chest revealed a moderately enlarged thymus gland unsuspected on a plain x-ray film. Myasthenia gravis was diagnosed on the basis of a high acetylcholine receptor antibody titer. Treatment started with prednisolone, anticholinesterase agent, and plasma exchange. The patient underwent a thymectomy 3 weeks after her diagnosis. Her symptoms were brought under control by anticholinesterase agent and prednisolone. This case illustrates the need to consider myasthenia gravis as well as other motor-neuron disorders when evaluating individuals presenting acute respiratory failure of unknown origin.