A 54-yr-old woman who presented with chest pain and elevated serum creatine kinase levels was found to have type III glycogen storage disease. Except for a history of hepatomegaly in childhood, she was healthy and lived a normal life. There was no hypoglycemia, seizure disorder or growth retardation. Muscle weakness was not apparent until the sixth decade. Despite the mild clinical course, debranching enzyme activity was not detectable by biochemical assay, and immunoblot analysis using a polyclonal antibody showed a complete absence of debrancher protein. Thus, mild clinical manifestations in this patient could not be explained by the residual debrancher enzyme and/or activity.