The development of hepatocellular adenomas and - more rarely - carcinoma in the liver of patients with Glycogen Storage Disease type Ia (GSDIa) is a well-known complication of the disease. The pathophysiology of adenoma and carcinoma development in these patients is, however, hitherto largely unknown and is thought to be related to the metabolic control of the patient and/or the type of mutations in the G6PC gene. We report here on a very illustrative case of adenoma and carcinoma formation in a previously undiagnosed 42 year old male GSDIa patient (enzymatically and genetically proven). He had two episodes of mild hypoglycaemia in childhood, never required formal treatment, showed normal growth, and only mild lactate increases after prolonged starvation. He was a long-distance runner for most of his adult life, without the need for more than normal carbohydrate intake before/during exertion. To gain a better view on the type of adenoma formed in this patient, molecular studies were performed. We show here that in this patient with mild GSDIa without recurrent hypoglycaemic episodes, adenoma and carcinoma formation still occurred and that malignant transformation of adenoma here is associated with CTNNB1 mutations and a typical mRNA profile of a beta-catenin activated lesion.