Five patients with adult onset acid maltase deficiency are described. All patients had developed their initial pelvic girdle symptoms late in the second or early in the third decade and some years later developed signs of respiratory insufficiency. Typically they were tall, had weak and wasted paraspinal and gluteal muscles with lower limb weakness. All were orthopnoeic with marked diaphragmatic weakness. In all patients characteristic glycogen vacuoles were seen in peripheral blood lymphocytes. Muscle biopsy was characteristic in only 1 out of 4 cases and showed minimal or nonspecific features in 3. The diagnosis was confirmed by enzyme assay in muscle and cultured fibroblasts. Long-term domiciliary ventilatory support (25 patient-years) using a rocking bed or intermittent positive pressure respiration with a tracheostomy has allowed a return to work in all patients, with their clinical condition remaining stable.