Linoleic acid [CH(CH)(CH = CHCH)(CH) COOH] is a precursor of the icosanoids -20-carbon fatty acids which include leukotrienes, prostaglandins, thromboxanes and related compounds. Until recently, the classical symptoms resulting from deficiency of linoleic and other essential fatty acids (EFAs) have been unexplained. Recently, however, the finding that linoleic acid is an important constituent of epidermal sphingolipids has suggested a biochemical mechanism for some of the EFA-deficiency skin symptoms and has indicated why linoleic acid is an essential component of the mammalian diet.