A 26-year-old woman presented with elevated liver enzymes, which were diagnosed two months ago. Examination revealed mild proximal muscle weakness, though the patient herself did not realise any impairment. The abdominal ultrasound and the histology of the liver remained unsuspicious. Muscle biopsy showed vacuolar degeneration, which could be ultrastructurally identified as large deposits of membrane-bound glycogen. The morphological findings prompted biochemical investigations which showed an excess of muscle glycogen. Acid maltase activity was reduced to < 10 % of normal, leading together with the clinical findings to the diagnosis of glycogenosis type II (Pompe's disease) of the adult type. Because of the modest impairment of the patient and the limited therapeutic possibilities, the patient remained thus untreated for.