Two men and one woman with debrancher deficiency had symptoms and signs of neuromuscular disease. The two men had adult-onset and slowly progressive weakness, distal muscle wasting, "mixed" electromyographic patterns, and slow nerve conduction velocities; the initial diagnosis was Charcot-Marie-Tooth disease in one patient and motor neuron disease in the other. The woman had stunted growth, delayed motor milestones, and lifelong nonprogressive weakness. A muscle biopsy specimen showed severe vacuolar myopathy in all three cases. The glycogen concentration was increased threefold to sixfold and had an abnormal iodine spectrum. Anaerobic glycolysis in vitro showed impaired use of endogenous and exogenous glycogen but normal use of hexose-phosphate glycolytic intermediates. These three cases illustrated the clinical variety of neuromuscular disease in debrancher deficiency. In patients with weakness of adult onset, the diagnosis is impossible to make without performing a muscle biopsy.