In a 48-year-old female, the first symptoms apparently manifested themselves 18 years before, with occasional tripping and weakness in both legs. During the next 18 years, weakness progressed and the patient developed a waddling gait; she became unable to rise from a lying or seated position unassisted and the shoulder girdle also became affected. Neurological examination revealed limb and shoulder girdle predominantly involving the lower extremities. We established cell cultures from muscle biopsy specimens obtained from our patient and carried out morphological analysis which, although aspecific, demonstrated clear signs of neurogenic suffering. This was confirmed in EMG studies performed. Biochemical analysis revealed very low acid maltase residual activity. We describe an unusual case of adult-onset acid maltase deficiency (AMD) with neurogenic atrophy and low residual activity. Innervated myofibres prepared by co-culturing the patient's myoblasts, with spinal cord foetal mouse explants were not associated with an abnormal in vitro maturation of the innervated myofibres as expected by the very low residual enzymatic activity found both in the muscle biopsy specimens and in the muscle cultures. There is strong suggestion that factors other than the amount of residual activity must be involved to determine the clinical manifestation of this disease.