Glycogen storage disease type III (GSD-III) or Cori/Forbes disease, is caused by autosomal recessive mutations in the AGL gene, which codes for the glycogen debranching enzyme (GDE) involved in the release of glucose-1P from glycogen branches. Abnormal glycogen accumulation is responsible for frequent hypoglycaemia and symptoms in the liver and striated muscles (GSD-IIIa), although some patients present with liver involvement only (GSD-IIIb). In childhood, the phenotype is mainly characterised by hepatomegaly, short stature and hypoglycaemia, with minimal skeletal muscle involvement. While liver symptoms improve spontaneously around puberty, skeletal muscle weakness develops progressively in adulthood and becomes a major feature of GSD-IIIa. Currently, there is no treatment other than dietary management tailored to the individual to limit glycogen storage and avoid hypoglycaemia. The French GSD-III registry is a multicentre online registry dedicated to patients with type III glycogen storage disease followed in France. It has been approved by ethical and regulatory authorities. Its main inclusion criteria is the presence of a proven pathogenic AGL gene mutation and/or reduced glycogen debranching enzyme activity. The aims of the registry are to provide a tool for recording detailed diagnostic, metabolic, neurological, cardiac and biological data on French patients with GSD-III, so as to enable i) a precise natural history of the disease, ii) identification of the outcome measures most sensitive to disease progression, iii) assessment of the frequency of the various complications of the disease and iv) identification of prognostic factors.