The case of a Japanese sialidosis type I patient with a novel NEU1 gene mutation is described. The patient developed an unsteady gait at age 14 and was referred to our hospital at age 16. On admission, subnormal intelligence, dysarthria, myoclonus, intentional tremors, limb and gait ataxia, hyperreflexia and macular cherry-red spots were observed. An enzymological analysis revealed a primary deficiency of neuraminidase. An NEU1 gene analysis identified two heterozygous missense mutations: p.P80L and p.D135N. The p.D135N mutation is a novel mutation that is considered to be associated with the mild clinical phenotype of sialidosis. Serial brain MRI showed diffuse brain atrophy progressing rapidly over the 41-month observation period.