Types A and B Niemann-Pick disease (NPD) result from the deficient activity of acid sphingomyelinase (ASM; E.C. 3.1.4.12) and the resultant lysosomal accumulation of sphingomyelin. Type A disease is a fatal, neurodegenerative disorder of infancy, whereas type B disease has no neurologic manifestations and is characterized primarily by reticuloendothelial involvement and survival into adulthood. Both disorders occur more frequently among individuals of Ashkenazi Jewish ancestry than in the general population. Recently, a missense mutation in the ASM gene (designated R496L) was detected in more than 30% of the ASM alleles from Ashkenazi Jewish type A NPD patients. We report a second, common mutation that resulted from a T to C transition at nucleotide 905 and predicted a leucine to proline substitution at ASM codon 302 (designated L302P). Notably, the L302P mutation occurred in 23.5% (8 of 34) of the Ashkenazi Jewish type A NPD alleles studied. In contrast, it was not found in any of the ASM alleles from non-Jewish type A patients, in 36 alleles from type B patients, or in 100 ASM alleles from normal Ashkenazi Jewish individuals. To confirm the authenticities of the L302P and R496L mutations, each nucleotide change was separately introduced into the full-length ASM cDNA by site-directed mutagenesis and transiently expressed in COS-1 cells. Neither mutation expressed ASM catalytic activity, consistent with the type A phenotype of homoallelic patients. The identification of the L302P mutation should further facilitate molecular carrier detection for NPD in the Ashkenazi Jewish population, particularly because the L302P mutation can be easily detected using the restriction enzyme, AlwNl.