This disclosure relates to mRNA therapy for the treatment of glycogen storage disease type 1a, (GSD-Ia), and related symptoms such as hypoglycemia. mRNAs for use in the invention, when administered in vivo, encode human glucose-6-phosphatase (G6Pase or G6PC), and functional fragments and variants thereof. mRNAs of the invention are preferably encapsulated in lipid nanoparticles (LNPs) to effect efficient delivery to cells and/or tissues in subjects, when administered thereto. mRNA therapies of the invention increase and/or restore deficient levels of G6PC expression and/or activity in subjects. mRNA therapies of the invention further increase the glucose production, and reduce the abnormal accumulation of glycogen and/or glucose-6-phosphate associated with GSD-Ia.