Glycogen storage disease type I (GSD I) is caused by a deficiency in one of the components of the glucose-6-phosphatase (G-6-Pase) system. Most patients have deficient G-6-Pase activity (GSD Ia) or deficient G-6-Pase translocase activity (GSD Ib). Both of these disorders result in hypoglycaemia, hepatomegaly, lactic acidaemia, hyperlipidaemia and hyperuricaemia, with additional findings of neutropenia in GSD Ib. Long-term complications include growth retardation, gout, hepatic adenomas, osteoporosis and renal disease (Chen and Burchell 1995). A female patient with GSD Ia identified with pulmonary hypertension, adding to the five previously described in the literature, prompted us to conduct a prospective study of the incidence of pulmonary hypertension in these patients.