In this report, the histologic criteria for the diagnosis of type Ia glycogen storage disease (GSD) in a wide age range were studied. Liver needle biopsies of 44 patients with type Ia GSD confirmed by enzyme analysis were re-evaluated and compared. Fatty change, nuclear hyperglycogenation (NH), and fibrosis were examined and graded. The second biopsies of 14 patients were also evaluated and compared with the first ones. The patients were grouped according to age: group I: <1 year (18 cases), group II: 1-5 years (19 cases), group III: >5 years (7 cases). A mosaic pattern was detected in all biopsies. The amount of fibrosis in group I was less than that in the other two groups. The fatty change in group I was more prominent. There was not much difference in the amount of NH between age groups. In comparing the two different biopsies of 14 patients, the amount of fibrosis was found to be increased in 7 cases. NH was also increased in a different group of 7 patients. These findings were both statistically significant. The amount of fatty change was minimal in most of the cases. Fibrosis is associated with types III, IV, VI, IX, and X GSD. Our results support previous studies stating that fibrosis may also be present and varies in extent in type I GSD. Fatty change as large lipid vacuoles and NH may not be seen in many cases of type I GSD. Therefore, histologic criteria for the diagnosis of GSD may not be specific, and enzyme analysis should be performed.