Two cases of pulmonary hypertension associated with type I glycogen storage disease (type I GSD) are reported. Before the development of pulmonary hypertension, patient 1 had been treated with dietary therapy with nocturnal gastric-drip infusion and zyloric therapy. Patient 2 had received a shunt operation between the intestinal vein and inferior vena cava, as well as dietary therapy. Both patients died of progressive heart failure due to pulmonary hypertension despite many attempts at drug therapy. There was no evidence in either case of a disorder that could have been the cause of the pulmonary hypertension. In case 1, the autopsy revealed a vasoconstrictive type of pulmonary hypertension with plexiform vascular lesions.