Sphingomyelins have been isolated in almost quantitative yield from normal and pathological human nervous tissues, and their fatty acid compositions determined by gas-liquid chromatography. In normal frontal lobe the proportion of stearic acid (18:O) decreases with increasing age from about 80% in the newborn to about 40% in the adult, whereas the C ~ Z C ~ ~ acids increase from about 10 to about 50%. In dysmyelinating diseases or malformations of the nervous system the content of C22-C26 acids was much smaller than in normal brains of the same age. In normal cortex 18:O constitutes at least two-thirds of the sphingomyelin fatty acids at all ages. In normal white matter from adults CZZ-CZ~ acids represent two-thirds of the acids present. We conclude that sphingomyelin of cytoplasm and that from myelin sheath show striking differences in the chain-lengths of their fatty acids. In patients who had died from dysmyelinating and demyelinating diseases the deviation from the normal pattern was much more pronounced in cerebral white matter than in total brain. Sphingomyelins of spinal medulla have a fatty acid pattern similar to that of adult brain but contain relatively higher amounts of 18 : 0 and 24 : 1. Sphingomyelins of peripheral nerve have a distinctly different fatty acid pattern, with much less 18 : 0 than in cerebral white matter.