Three Siamese cats were found to have a progressive neurological disease that became obvious when they were 4 to 5 months of age. Their brains contained an excess of GM2 and GM3 gangliosides, and their livers a nine- to tenfold excess of sphingomyelin and cholesterol. A total deficiency of lysosomal (pH 5.0) sphingomyelinase was found in the leukocytes, liver, and brain of the cats, although the activity of the microsomal (pH 7.4, magnesium-dependent) sphingomyelinase was normal in brain. These cats appear to have a genetic disease identical to Niemann-Pick disease type A.