Thalamic hyperdensity on computer-associated tomography (CT) has been reported as a disease-specific finding for GM2-gangliosidosis. However, a boy with the typical clinical symptoms and enzyme defect of infantile GM1-gangliosidosis had hyperdense thalamic lesions at 12 months of age. At the age of 34 months, the thalamus showed atrophy and the hyperdensity was restricted to the medial part of the thalamus, which exhibited increased intensity on T1-weighted and decreased intensity on T2-weighted images. Thalamic hyperdensity may be specific not only to GM2-gangliosidosis but also to GM1-gangliosidosis.