Niemann-Pick Disease, Types A (neurodegenerative form) and B (visceral form) are rare allelic lipid storage disorders due to mutations in the SMPD1 gene and are characterized by accumulation of sphingomyelin in reticulo-endothelial and other cell types in the body. Niemann-Pick Disease, Type A is panethnic but especially common in the Ashkenazi Jewish population. It is more severe and progressive than Type B because the SMPD1 gene product, acid sphingomyelinase, is completely abolished or enzyme activity is < 5%. The disorder usually presents early with abdominal enlargement due to hepatosplenomegaly. Starting at around 6 months of age, other symptoms follow, including persistent jaundice, cherry red spots of the retina, hypotonia, progressive growth, motor and developmental delay with failure to achieve developmental milestones such as independent sitting, crawling, or walking. The disorder then results in rapid neurological degeneration, hypotonia, rigidity, and mental retardation, with fatal outcome within approximately 3 years after onset. Niemann-Pick Disease, Type B is panethnic, although increased frequency in Turkish, Arabic and North African populations due to founder mutations has been noted. The clinical phenotype of Niemann-Pick Disease, Type B is less homogeneous than Type A, and symptoms may involve the spleen, liver, and lungs. Niemann-Pick Type B patients remain mostly free of neurological manifestations and typically live into adulthood. In Niemann-Pick Disease Type B, the defective enzyme retains residual catalytic activity, thus resulting in the milder phenotype. Intermediate Type A/B falls clinically on a continuum, with symptoms ranging from slightly less severe than Type A to slightly more severe than classic Type B.