Glycogen storage disease type I (GSD-I) is a group of autosomal recessive disorders with an incidence of 1 in 100,000. The two major subtypes are GSD-Ia, caused by a deficiency of glucose-6-phosphatase (G6Pase), and GSD-Ib, caused by a deficiency of glucose-6-phosphate transporter (G6PT). We report that a substantial improvement was achieved following several infusions of hepatocytes in a patient with GSD-Ib. Hepatocytes were isolated from the unused cadaveric whole livers of two donors. At the first transplantation, approximately 2 x 10(9) cells (2% of the estimated recipient's total hepatocytes) were infused. Seven days later 1 x 10(9) (1% of liver mass) cryopreserved hepatocytes from the same donor were infused, and an additional 3 x 10(9) (3% of liver mass) cells from the second donor were infused 1 month after the second transplantation. After the hepatocyte transplantation, the patient showed no hypoglycemic symptoms despite the discontinuation of cornstarch meals. Liver biopsies on posttransplantation days 20 and 250 showed a normal level of glucose-6-phosphatase activity in presolubilization assay that was very low before transplantation. This was the first and successful clinical hepatocyte transplantation in Korea. In this study, hepatocyte transplantation allowed a normal diet in a patient with GSD-Ib, with substantial improvement in their quality of life. Hepatocyte transplantation might be an alternative to liver transplantation and dietary therapy in GSD-Ib.