PURPOSE: Glycogen storage disease type III is a rare disease of variable clinical severity affecting primarily the liver, heart, and skeletal muscle. It is caused by deficient activity of glycogen debranching enzyme, which is a key enzyme in glycogen degradation. Glycogen storage disease type III manifests a wide clinical spectrum. Individuals with glycogen storage disease type III present with hepatomegaly, hypoglycemia, hyperlipidemia, and growth retardation. Those with type IIIa have symptoms related to liver disease and progressive muscle (cardiac and skeletal) involvement that varies in age of onset, rate of disease progression, and severity. Those with type IIIb primarily have symptoms related to liver disease. This guideline for the management of glycogen storage disease type III was developed as an educational resource for health care providers to facilitate prompt and accurate diagnosis and appropriate management of patients. METHODS: An international group of experts in various aspects of glycogen storage disease type III met to review the evidence base from the scientific literature and provided their expert opinions. Consensus was developed in each area of diagnosis, treatment, and management. RESULTS: This management guideline specifically addresses evaluation and diagnosis across multiple organ systems (cardiovascular, gastrointestinal/nutrition, hepatic, musculoskeletal, and neuromuscular) involved in glycogen storage disease type III. Conditions to consider in a differential diagnosis stemming from presenting features and diagnostic algorithms are discussed. Aspects of diagnostic evaluation and nutritional and medical management, including care coordination, genetic counseling, hepatic transplantation, and prenatal diagnosis, are addressed. CONCLUSIONS: A guideline that will facilitate the accurate diagnosis and appropriate management of individuals with glycogen storage disease type III was developed. This guideline will help health care providers recognize patients with all forms of glycogen storage disease type III, expedite diagnosis, and minimize stress and negative sequelae from delayed diagnosis and inappropriate management. It will also help identify gaps in scientific knowledge that exist today and suggest future studies.