Congenital methemoglobinemia is caused by NADH-methemoglobin reductase deficiency in more than half of the total reported cases. NADH-methemoglobin reductase deficiency is an uncommon hereditary disorder producing methemoglobinemia and cyanosis in the homozygous subject. A majority of the patients born with these abnormalities have only a cosmetic defect-asymptomatic cyanosis. Congenital methemoglobinemia due to NADH-methemoglobin reductase deficiency is an autosomal recessive disorder and classified into 4 types according to the pathophysiology of the disorder. In type I, the deficiency of NADH-methemoglobin reductase is restricted to erythrocytes of patients with mild cyanosis, and 7 missence mutations have been reported in the case of type I. We report the first Korean pediatric case of type I congenital methemoglobinemia due to NADH- methemoglobin reductase deficiency with a review of the literature.