Lee YM 1 , Kang HC 1 , Lee JS 1 , Kim HD 1 1 Dept of Ped, Yonsei Univ Col Med, Seoul, Korea, Republic of Tay-Sachs disease (TSD) is a metabolic disease, inherited in an autosomal recessive way, that is caused by a defect of hexosaminidase A. The gene responsible for the disease, HEXA gene, is located on chromosome 15q23-q24. Though TSD is rare in the general population, the frequency is reported to be 100 times higher in the Ashkenazi Jewish and eastern Quebec French Canadian. The disease is characterized by progressive weakness, loss of motor skills, decreased attentiveness, and increased startle response with progressive evidence of neurological degeneration. Hexosaminidase A activity study was performed in a boy who had shown delayed development since 6 months of age, regression since 9 months and exaggerated startle response. He had no unusual history nor medical event at birth. His hexosaminidase A activity was decreased and progressive brain atrophy was noted in serial brain MRI. HEXA gene study was performed and direct sequencing of the amplified genomic DNA revealed 2 kinds of mutations being in compound heterozygosity (c.913_915delTTC and IVS5-1 G>T). Family members received HEXA gene study and all were diagnosed as carriers. We would like to report the very first case of the classic infantile form of TSD that was confirmed by HEXA gene study in Korea.