Sixteen patients with glycogen storage disease type III (GSD III) aged 3 to 22 years underwent a detailed neuromuscular evaluation. A minimal impairment of skeletal muscle function was presented in eight patients, slight impairment in four and severe impairment in one patient. Serum creatinine phosphokinase (CPK) was elevated in all patients studied. In the nine patients, in whom electromyography (EMG) was performed; six exhibited a myopathic pattern while a "mixed" (neurogenic-myopathic) pattern was present in three. Muscle biopsies performed in 12 patients, revealed in all cases amylo-1,6,-glucosidase deficiency and biochemical as well as morphological evidence of glycogen accumulation. Two brothers suffered from late onset myopathy, which in the older sibling was associated with clinical, EMG and EM findings of a peripheral neuropathy. Fifteen patients had either electrocardiographic and or echographic evidence of cardiomyopathy. Observations based on this patient material suggest a widespread myopathy in GSD III patients with heterogeneous expression, while peripheral nerve involvement is rarely encountered.