Introduction: Mucopolysaccharidosis type III (MPS III, Sanfilippo syndrome) is an autosomal recessive disorder caused by a deficiency in four enzymes involved in the lysosomal degradation of the glycosaminoglycan heparan sulfate. It is characterized by progressive neurocognitive decline, behavioral abnormalities, sleep disturbances and relatively mild somatic manifestations, and clinical manifestations are indistinguishable among the MPS III subtypes. The aim of this study was to evaluate the clinical and molecular genetic characteristics of 21 Korean patients with MPS type III. Methods: We retrospectively reviewed 21 patients from 20 unrelated families who were diagnosed as MPS III by enzyme assay from January 1997 to December 2012 in Samsung medical center. Clinical manifestations and results of molecular genetic analysis were investigated. Results: Ten patients(48 %) were diagnosed as MPS IIIa and MPS IIIb, respectively, and one patient( 4 %) as MPS IIIc. The median age of onset of first signs or symptoms was 3.3 years (range 1.5-5.0 ), and median age at diagnosis was 5.9 years (range 1.9-10.2 ). The average delay between the first presenting signs/symptoms and the diagnosis was 2.6 years(range 0.2-6.3 years). The most common first presenting signs/symptoms of 20 patients was developmental delay(95%), and only one patient showed hepatomegaly(5%). At the time of diagnosis, facial dysmorphism was observed in 17 patients(80%). The other clinical manifestations of patients who could be followed up were behavioral disturbances(67%), sleep disturbances(62 %), seizure(57 %), orthopedic problem(42%). The median age at loss ability to speech was 8 years (range 5.7-11 ) and those to walk was 9.9 years (range 7.9-13.0 ) in patients who were available in follow-up. Molecular genetic analysis was performed in 7 of the 21 MPS III patients, and 9 different sequence variants were identified including 1 nonsense, 1 splice site mutation and 7 missense mutations. Four novel mutations were detected. Conclusions: It is suggested that when a patient shows developmental delay and/or characteristic somatic feature of MPS , Screening for MPS III should be considered.