Pompe's disease (acid maltase deficiency) classically affects infants and children, with a few sporadic cases occurring in adults. An adult patient initially have progressive muscular weakness, exertional dyspnea, diaphragmatic paralysis, and objective evidence of restrictive respiratory disease. Muscle biopsy established the diagnosis of acid maltase deficiency. The patient's brother had died at the age of 44 years, after 23 years of a "progressive muscular dystrophy." Acid maltase deficiency should be considered in the differential diagnosis of progressive respiratory insufficiency associated with weakness.