Sialidosis (mucolipidosis type I) is a rare inherited lysosomal storage disease characterized by deficiency of α-N-acetylneuraminidase (sialidase) in leukocytes and cultured fibroblasts.1This results in intracellular storage of excess sialyloligosaccharides and is histologically observed as abnormal vacuolization of various cell types. Two major phenotypes of mucolipidosis exist: type I or the cherry red spot myoclonus syndrome, and a more severe infantile form, type II. Sialidosis has an autosomal recessive pattern of inheritance, and the gene has been localized on chromosome 6p21.2It occurs in 1 of every 2 200 000 live births. Patients with type II disease have somatic changes, characterized by coarse facies, hepatolomegaly, bony changes of dysostosis multiplex, and developmental delay. Patients with type II disease usually die within the first 2 years (congenital form) or by the second decade (infantile form) of life.3By contrast, patients with type I disease are less severely affected and typically develop symptoms of myoclonic epilepsy, visual problems, and ataxia in the second or the third decade of life.4Macular cherry red spots are always present and, therefore, sialidosis should be included in the differential diagnosis of a cherry red macula in this clinical setting. Diagnostic evaluation for a patient with a cherry red spot in the macula not due to arterial occlusion should include a genetic history and an appropriate laboratory workup to confirm the underlying cause.