Defects in neuraminidase activity (N-acetylneuraminic acid hydrolase EC 3.2. 1.18) have recently been described in several clinical disorders. Cantz et al.[1] and Spranger et al.[2] demonstrated a profound deficiency of acid neuraminidase in a patient who had been previously classified as having mucolipidosis I [3, 4]. The patient had a neurodegenerative disorder with Hurleroid features, skeletal dysplasia, and cherry-red spots in the macula. Signs of the illness developed in the first year of life. In addition to the neuraminidase defect, these patients accumulated sialicacid-containing material in fibroblasts. Parents had activities of neuraminidase which were intermediate between patients and controls, implicating the deficiency asa primary defect. Durand et al.[5], O'Brien [6], Thomas et al.[7], and Rapin et al.[8] have described a group of young adults with neuraminidasedeficiency who presented with decreasing visual acuity, cherry-red spots in the macula, myoclonus, and sialyloligosaccharides in the urine. Their phenotype differs from that of mucolipidosis I patients in that intelligence is normal, somatic andbony abnormalities are absent, and their survival is longer. Family studies indicate that patients with the" cherry-red spot-myoclonus" syndrome are homozygous for a recessive autosomal mutation affecting acid neuraminidase [6, 7]. An infant with Hurleroid features but normal mental development and acid neuraminidase deficiency has been described by Kelly and Graetz [9]. In addition to these two groups of patients another dysmorphic group of young adults, who were previously thought to have a primary defect in 18-galactosidase activity, has now been included in the clinical spectrum of patients with neuraminidase deficiency [10, 11]. Although these patients are normal at birth and throughout early childhood, dysmorphic features, psychomotor degenerative changes, myoclonus, and frequently a cherry-red spot develop at around 10 years of age. It is timely then to review the clinical phenotype of primary human neuraminidase deficiency. We employ the term" sialidosis" here to describe patients withprimary neuraminidase deficiency and believe it should be reserved exclusively for this condition. We also discuss the clinical genetics of sialidosis, the chemistry of the compounds stored, the molecular genetics of acid neuraminidase, the secondary neuraminidase deficiencies in other disorders, the secondary deficiency of f3-galactosidase in some patients with sialidosis, and laboratory methods for diagnosis.