Niemann-Pick disease (NPD) types A and B are autosomal recessive disorders caused by acid sphingomyelinase (ASM) deficiency due to mutation in the sphingomyelin phosphodiesterase 1 gene (SMPD1). Although a number of SMPD1 mutations were reported, expression studies were performed for only a small number of missense mutations. We evaluated three unrelated patients with clinical manifestations of NPD. Sequence analysis revealed two previously described (S248R and W391G) and two novel (G247D and F572L) missense mutations. To analyze the effects of the novel mutations on ASM function, cDNA was generated by site-directed mutagenesis and expressed in COS-7 cells. In vitro biochemical assays revealed marked deficiency of ASM activity consistent with the disease phenotype in cells homoallelic for each mutation. We show that each mutation dramatically reduced half-life and catalytic activity of ASM with more pronounced decrease by the G247D mutation. These data suggest that impaired protein stability and decreased enzyme activity are responsible for the disease in sphingomyelinase-deficient patients carrying the G247D and F572L mutations.