Glycogen storage disease (GSD) type Ia is rare inborn metabolic disorder, caused by glucose-6-phosphatase deficiency. It characterized by hepatomegaly, hypoglycemia, lactic acidosis, hypertriglyceridemia, and hyperuricemia and it is usually manifested in the infantile period. In addition, it is also associated with growth failure, pubertal delay, anemia, platelet dysfunction, osteopenia, and pulmonary hypertension. Hepatocellular adenoma and renal dysfunction are frequent late complications. Delayed diagnosis and inappropriate therapy lead to many complications such as growth failure, osteoporosis, refractory gout, renal failure, hepatocellular carcinoma (HCC), and pulmonary hypertension. Here, two Korean sisters diagnosed with GSD Ia, aged 33 and 36 respectively, were described and compared to recent articles about four adults with late diagnosis of GSD Ia. One sister had typical manifestations of GSD Ia including short stature (height, 145 cm), multiple hepatic adenoma, chronic kidney disease stage IV, and severe osteoporosis, whereas the older sister had normal stature (162 cm), one tiny hepatic nodule, and normal renal function. Direct sequencing of G6PC in two sisters identified a homozygous splicing mutation, c.645G>T, which is a prevalent mutation in Korea. Interestingly, our cases and four adults from recent reports had asymptomatic mild hypoglycemia and various manifestations including renal failure, HCC, fatty liver, or uncontrolled hyperlipidemia. These adult cases represent not only heterogenous phenotype to genotype within family members with GSD Ia but also long-term complications such as gouty arthritis, renal failure, and osteoporosis in untreated adult GSD Ia patients. In addition, lactic academia and hypertriglyceridemia are good markers of GSD Ia to distinguish from metabolic disease.