13-Hydroxyoctadecadienoic acid (13-HODE) is the commonly used term for 13(S)-hydroxy-9Z,11E-octadecadienoic acid. The production of 13(S)-HODE is often accompanied by the production of its stereoisomer, 13(R)-hydroxy-9Z,11E-octadecadienoic acid. The adjacent figure gives the structure for the (S) stereoisomer of 13-HODE. Two other naturally occurring 13-HODEs that may accompany the production of 13(S)-HODE are its cis-trans isomers viz., 13(S)-hydroxy-9E,11E-octadecadienoic acid and 13(R)-hydroxy-9E,11E-octadecadienoic acid. Studies credit 13(S)-HODE with a range of clinically relevant bioactivities; recent studies have assigned activities to 13(R)-HODE that differ from those of 13(S)-HODE; and other studies have proposed that one or more of these HODEs mediate physiological and pathological responses, are markers of various human diseases, and/or contribute to the progression of certain diseases in humans. Since, however, many studies on the identification, quantification, and actions of 13(S)-HODE in cells and tissues have employed methods that did not distinguish between these isomers, 13-HODE is used here when the actual isomer studied is unclear.