The present article analyzes the symptoms of essential fatty acid deficiency, both in human beings and in animals. The first part of the article describes the interrelationships between linoleic, linolenic and arachidonic acids, and how these acids affect the physiological response in experimental animals. Likewise, it discusses the effect of gradual levels of linoleic acid intake, and its use to establish quantitative requirements for essential fatty acids on the basis of biochemical parameters. It provides information also on the effect of dietary non-essential fatty acids on essential fatty acid metabolism. The second part of the article deals with the significance of essential fatty acids in human nutrition. In spite of the available information in this regard, it is suggested that further and more thorough studies be conducted. Data on human blood serum fatty acid composition are presented, which could be used as a basis for comparison when essential fatty acid deficiency is suspected. The author further indicates that there are several dietary factors that could induce essential fatty acid deficiency, or that could result in marginal states of deficiency in these nutrients.