A 21-year old women with rhizomelic muscular deficit and signs of hypercapnia developed acute respiratory failure. Laboratory tests revealed high creatine kinase activity, and electromyograms showed myogenic patterns with a few myotonic discharges. Biopsy of the quadriceps muscle elicited major vacuolar myopathy with glycogen overload. Acid maltase activity was undetectable in muscular tissue. After 7 months on high-protein diet (1540 calories, 37% proteins) there was no clinical or biochemical improvement. The other published cases of acid maltase deficiency treated with high-protein diet are discussed.