The fatty acids composition of cerebrospinal fluid in hydrocephalic patients was analyzed by gas chromatography and fatty acid content was determined using arachidic acid as an indicator. In 40 cases of congenital hydrocephalus, total fatty acid values were found to be significantly higher than normal in the preoperative period. After shunt operation, hydrocephalus came to a standstill in 6 cases. They were not dependent on the shunting system. Their quantitative fatty acid composition was maintained within normal limits after removal of the shunting system.