A 51-year-old man presented with a six-year history of gradually progressive exertional dyspnea. His complaint was attributed to increasing age, but evaluation finally led to a diagnosis of diaphragmatic paralysis. Neurological examination disclosed only minimal evidence of weakness of limb muscles. Electromyographic studies suggested a diagnosis of acid maltase deficiency, which was confirmed by electron microscopic and biochemical studies of muscle biopsy sections. Exercise stress testing was necessary to define the limits of the patient's exercise tolerance.