Glycogen storage disease type Ia (GSDIa) is a rare genetic disorder of carbohydrate metabolism caused by a deficiency of glucose-6-phosphatase that can lead to acute and chronic complications, including life-threatening hypoglycemia, metabolic abnormalities, growth failure, renal dysfunction, and hepatocellular carcinoma. Other than dietary management with cornstarch, there are no treatments approved for GSDIa. However, promising therapies are in development, which may require health technology assessments. We aimed to develop and validate a conceptual cost effectiveness model that captures both short- and long-term manifestations of GSDIa, including potential impacts of future treatments.