PROBLEM TO BE SOLVED: To provide a stress/fatigue marker and a method for evaluating stress/fatigue.SOLUTION: A stress and/or fatigue marker is selected from a group consisting of 12-HETE (12-hydroxy- 5, 8, and 10, 14-eicosatetraenoic acid), 15-HETE (15-hydroxy-5, 8, 11, 13-eicosatetraenoic acid), and 13-(Z, E)-HODE (13-hydroxy-9 (Z) 11 (E)-octadecadienoic acid).