Sialic acid storage disease, also known as Salla disease, is a lysosomal storage disorder caused by defective transport of lysosomal degradation product, free sialic acid, across the lysosomal membrane. Functional consequences come only from central nervous system presenting slowly progressive mental retardation associated with ataxia and some other neurological manifestations. Affected infants present first signs of the disease already at 3 to 9 months of age as muscular hypotonia, truncal and limb ataxia, often transient nystagmus and delayed motor development. Developmental profile of SD patients is characterized by slowly progressive general handicap with motor performance more severely and earlier affected than cognitive skills. All patients are severely mentally retarded from third decade on. Life span of affected patients is close to normal.