A 4.5 months old girl was suspected to have Glycogenosis type I because of hepatomegalie and recurrent hypoglycemia. Liverbiopsy revealed a normal glycogen content and a normal in vitro activity of glucose-6-phosphatase. We then examined the carbohydrate metabolism and could demonstrate that in vitro the transfer of glucose-6-phosphate to glucose was blocked. We therefore conclude that a normal in vitro activity of glucose-6-phosphatase does not rule out the diagnosis of Glycogenosis type I. Evaluation of carbohydrate metabolism is an important tool in marking the diagnosis. We suggest to use the term Glycogenosis type I B, which some institutions already use for this disorder.