Glycogen storage disease type Ia (GSD‐Ia) is an autosomal recessive disorder of glycogen metabolism caused by a deficiency of glucose‐6‐phosphatase (G6Pase) that is expressed in the liver, kidney, and intestinal mucosa. Clinical manifestations include short stature, hepatomegaly, hypoglycemia, hyperuricemia, and lactic acidemia. To elucidate a spectrum of the G6Pase gene mutations and their frequencies, we analyzed mutations in 51 unrelated Japanese patients with GSD‐Ia. The most prevalent mutation was g727t, accounting for 88 of 102 mutant alleles examined, followed by R170X mutation, which accounted for 6 mutant alleles, and R83H mutation which was observed in 3 mutant alleles. In addition, 3 different, novel mutations, IVS1‐1g<a, Gly122‐to‐Asp (G122D) and His179‐to‐Pro (H179P), were identified. We were able to detect “ectopically” transcribed G6Pase‐mRNA in Epstein‐Barr virus‐transformed lymphoblastoid cells and observed aberrant mRNA splicing associated with the g727t and IVS1‐1g