Introduction : Tay-Sachs disease is a rare genetic disorder with autosomal recessive inheritance. It is caused by mutation in HEXA (hexoaminidase A) gene and characterized by progressive deterioration of mental and physical abilities and results in death. We report a case of confirmed Tay-Sachs disease with unusual initial presentation. Case report : The patient is a 4 years 1 month old boy with unremarkable antenatal and perinatal history. He had strabismus at 1 year, and obtained brain MRI (magnetic resonance imaging) scan which revealed arachnoid cyst of posterior fossa. He started to walk on own at age 15 months and was able to say few words. However, from the age of 24 months, he started to decline with losing ability to talk as well as worsening gait, and was seen by a child neurologist at Ilsan Hospital. Repeat brain MRI scan was unremarkable, but he continued to decline. He was once again seen by a child neurologist at Seoul National University Hospital and genetic mutation testing for metachromatic leukodystrophy was negative. In between, he started to have seizures 2-3 times daily for about 20 seconds each as well as startling response upon noises, for which he visited our child neurology clinic. Even though he did not show cherry-red spot of usual Tay-Sachs disease, for his unusual symptoms of startling response upon clapping noise in the clinic, we sent genetic mutation testing for HEXA gene which came back positive. Currently, at the age of 4 years 1 month, he continues to decline with inability to sit, babble or swallow. Discussion : Our case warrants a consideration of Tay-Sachs disease when evaluating a patient with known progressive neurological deficit, epilepsy and startling response upon noises even in absence of cherry-red spot.