PURPOSE: To describe the clinical progression and ophthalmic findings in a Japanese boy with Tay-Sachs disease at ages 5 and 8 months. RESULTS: The patient was born at 38 weeks of gestation and developed normally until motor skill delays were identified at age 1 year and 1 month. At that time, brain magnetic resonance imaging revealed diffuse T2 hyperintensity in the bilateral basal ganglia. At age 1 year and 6 months, he was able to fix and follow objects, and the optic discs appeared normal, but bilateral cherry-red spots were observed in the maculae. Based on these findings, together with hypersensitivity to sound and markedly reduced β-hexosaminidase A activity, Tay-Sachs disease was biochemically diagnosed. By age 2 years and 1 month, the patient had lost the ability to fixate on and follow objects. Fundoscopy at age 4 years and 4 months revealed optic atrophy and reduced cherry-red spots, electroretinography (ERG) at age 4 years and 9 months showed a complete loss of retinal responses, and optical coherence tomography at age 5 years and 3 months demonstrated retinal thinning. Long-term observation revealed progressive degeneration, consistent with previous reports. Lipid deposition in all retinal layers was considered to contribute to retinal atrophy. CONCLUSIONS: This report highlights progressive retinal degeneration with optic atrophy, retinal thinning, and reduction in cherry-red spots. The flat ERG waveform in this patient suggests more advanced retinal involvement than in previous reports and provides insights into the ocular manifestations of Tay-Sachs disease.