A 40-year-old man suffered for 5 years from a progressive proximal myopathy mimicking an atypical limb-girdle dystrophy. A "myopathic" pattern with myotonic and pseudomyotonic discharges was determined by electromyography. Enzyme histochemical and ultrastructural investigations of muscle and liver biopsies pointed to a glycogenosis. Biochemical investigations of muscle and liver samples confirmed this diagnosis, disclosing an acid maltase deficiency. Glycogen filled lysosomes were also revealed electron optically in skin fibroblasts but not in white blood cells. The literature concerning the late onset forms of acid maltase deficiency (type II glycogenosis) has been reviewed, and the clinical course has been compared with that of the infantile form (Pompe's disease). In early infancy the disease has a short and fatal course, with involvement of many organs. primarily skeletal muscules, liver and heart. In the late infantile and juvenile forms the course of the disease is slower, the organ involvement beeing not as severe; muscular symptoms begin to prevail. In adults, type II glycogenosis mimics muscular dystrophy with its prolonged course and the almost exclusive clinical involvement of proximal muscles. Biochemical and ultrastructural investigations have nevertheless demonstrated that other organs and tissues are also involved. The reasons for the variability of organ involvements in different ages are as yet unknown.