We report on two cases of type I glycogen storage disease (GSD) complicated by malignant tumors. A 23-year-old man had GSD Ia with adenomatosis. He underwent transplantation for rapidly growing and radiologically changing adenomata. At histological examination, one adenoma had become a hepatocellular carcinoma. A 22-year-old, HBV-infected woman had GSD type Ib with adenomatosis. At follow-up, several tumors showed changing morphological characteristics. Pre-transplant laparotomy confirmed the presence of a metastatic cholangiocarcinoma. Liver transplantation should be considered in GSD type I patients with adenomatosis, especially when tumor characteristics change. Regular detailed Doppler ultrasound and magnetic nuclear resonance screening during childhood and adolescence are, therefore, mandatory in order for the timing of transplantation to be optimized.