Fructose-1,6-bisphosphatase (FBPase) deficiency is an autosomal, recessively inherited disease that progresses with severe hypoglycemia, and metabolic attacks result in a defect in gluconeogenesis. If not appropriately treated, and if fructose is not excluded from the diet, the outcome could be fatal. Two Turkish children with FBPase deficiency were diagnosed based on mutation of the FBP1 gene. The first, a 2-year-old girl, was referred to our clinic because of lactic acidosis, uncorrectable hypoglycemia, and increased transaminases. FBPase deficiency was suspected in the patient, who recovered dramatically after a high-dose glucose infusion and adequate bicarbonate replacement. The second patient, a five-and-a-half-year-old male sibling of the patient, was also hospitalized, twice, because of hypoglycemic attacks and metabolic acidosis. Different from previous analyses, a homozygous c.658delT mutation was detected at exon 5 of the FBP1 gene in the two siblings. As a result of this mutation, there was a TGA (stop codon) at exon 6. There was first-degree consanguinity between the parents. These two cases were the first FBP1 gene mutations reported in our country.