The lysosomal free sialic acid storage diseases (SASDs) include the allelic disorders Salla disease (SD), common in the Finnish population, and infantile free sialic acid storage disease (ISSD). Patients with SD usually have a normal appearance and neurological findings at birth followed by slowly progressive neurologic deterioration resulting in moderate to severe psychomotor retardation, spasticity, ataxia and seizures. The life expectancy appears to be shortened; however, some affected patients have been reported to live into their early seventies. ISSD has a much more severe phenotype characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and failure to thrive. Cardiomegaly, renal involvement and dysostosis multiplex may also be present. ISSD can present prenatally or in the neonatal period with non-immune hydrops fetalis and/or isolated ascites. Death usually occurs in early childhood. Cases of SASD have been reported with symptoms that are intermediate between SD and ISSD. SD has mostly been reported in individuals from Finland, where a p.Arg39Cys (R39C) founder mutation in the SLC17A5 gene was identified in approximately 1 in 200 individuals. In the northeastern region of Finland where SD is more common, the carrier frequency of R39C is approximately 1 in 100.