Maturity-Onset Diabetes of the Young (MODY) refers to a heterogeneous group of monogenic diabetes. Unlike other types of MODY characterized by genetic defects in transcription factors, MODY 2 is triggered by metabolic alterations caused by mutations of glucokinase (GCK), the first enzyme of the glycolytic pathway. We report a three-generation Chilean family with multiple cases affected with this disease. The index case is a patient who presented severe neonatal hyperglycemia (831 mg/dl, without ketosis) requiring continuous infusion of insulin, which was suspended after 48 hours with normalization of blood glucose. Subsequently, continuous glucose monitoring at 4 months of age revealed 47% of tissue glucose levels above 140 mg/dl, with fasting glucose levels between 120 and 166 mg/dl. The genetic analysis revealed a previously reported mutation in heterozygous state of the GCK gene (c.148C> T; p.His50Tyr). This mutation was also identified in more than one affected relative in the last two generations, with a transmission pattern suggestive of dominant inheritance. GCK gene sequencing led to a correct molecular diagnosis of MODY 2 while bioinformatic analysis indicated the possible molecular causes of the enzyme dysfunction. The knowledge of the molecular diagnosis allowed an adequate medical treatment for this disease.Maturity-Onset Diabetes of the Young (MODY) refers to a heterogeneous group of monogenic diabetes. Unlike other types of MODY characterized by genetic defects in transcription factors, MODY 2 is triggered by metabolic alterations caused by mutations of glucokinase (GCK), the first enzyme of the glycolytic pathway. We report a three-generation Chilean family with multiple cases affected with this disease. The index case is a patient who presented severe neonatal hyperglycemia (831 mg/dl, without ketosis) requiring continuous infusion of insulin, which was suspended after 48 hours with normalization of blood glucose. Subsequently, continuous glucose monitoring at 4 months of age revealed 47% of tissue glucose levels above 140 mg/dl, with fasting glucose levels between 120 and 166 mg/dl. The genetic analysis revealed a previously reported mutation in heterozygous state of the GCK gene (c.148C> T; p.His50Tyr). This mutation was also identified in more than one affected relative in the last two generations, with a transmission pattern suggestive of dominant inheritance. GCK gene sequencing led to a correct molecular diagnosis of MODY 2 while bioinformatic analysis indicated the possible molecular causes of the enzyme dysfunction. The knowledge of the molecular diagnosis allowed an adequate medical treatment for this disease.