Diseases of inborn errors of metabolism (IEMs) are very rare but the overall prevalence of IEMs is not low, and in the United States, about 5~10% of admitted patients have some genetic predispositions. Clinical manifestations of IEMs are very diverse, but most frequent manifestations are neurological symptoms and signs. IEMs in Korea have been underestimated because of prejudice, underdevelopment of diagnostic tools and ignorance. The Korean Pediatric Society has done a retrospective study in order to know the relative incidence of IEMs in 2001. All hospitals with over 100 beds participated in the study. The most frequent disease was Wilson disease (201 cases for 10 years) followed by phenylketonuria (98 cases for 10 years) and Hunters disease (69 cases for 10 years). Disorders of mineral metabolism were the most frequently diagnosed disease groups (252 cases for 10 years) followed by organic acidopathies (220 cases), aminoacidopathies (139 cases), mucopolysaccharidosis (131 cases), disorders of carbohydrate metabolism (84 cases), sphingolipidosis (69 cases), urea cycle disorders (39 cases), peroxisomal disorders (27 cases), porphyrias (16 cases), disorders of purine and pyrimidine metabolism (14 cases), disorders of membrane transport (13 cases), fatty acid oxidation disorders (9 cases), oligosaccharidosis (2 cases), and mucolipidosis (1 case). Clearly, Koreans are not protected from IEMs and a systematic approach is needed to make diagnosis more easy and accurate.