PURPOSE: Glycogen storage disease type Ia (GSD Ia) is an autosomal recessive disorder caused by the deficiency of glucose-6-phosphatase (G6Pase). The aim of the study was to investigate the spectrum of G6Pase gene mutations and relationship between genotype and clinical findings in Korean patients with GSD Ia. METHODS: Genomic DNA was extracted from peripheral leukocytes of 20 patients with GSD Ia. The five exons of G6Pase gene were amplified and PCR products were directly sequenced. The frequency of short stature, hypoglycemia, hypercholesterolemia, hyperuricemia, hypercalciuria, nephrocalcinosis and hepatic adenoma was compared between 727G> T homozygotes and 727G> T compound heterozygotes. RESULTS: A total of 5 different mutations were identified. The most common mutation was the 727G> T with an allele frequency of 80%. All patients were either homozygous (12/20) or heterozygous (8/20) for the 727G> T mutation. G122D was found in 3 patients, P178A in 1, G222R in 2, and S339R in 2. There was no difference in the frequency of short stature, hypoglycemia, hypercholesterolemia, hyperuricemia, nephrocalcinosis, and hepatic adenoma between 727G> T homozygotes and heterozygotes. CONCLUSION: Diagnosis of GSD Ia can be based on clinical and biochemical abnormalities combined with mutation analysis instead of enzymatic diagnosis that requires liver biopsy. Homozygosity for the 727G> T does not seem to alter the disease phenotype as compared with the heterozygous state.