SummaryBackgroundThere are two types of damage pattern of human hair cuticle: type L, where the cell membrane complex is split and the cuticle lifts up, and type E, where the fragile substructure of the cuticle cell (endocuticle) is broken. In our previous paper, it was reported that the dominant damage pattern shifts from type L to E with the subjects' age around the 40s. Loss of the cuticle due to daily grooming stresses increases with the subjects' age and is related to the level of type E damage. It is supposed that deterioration of endocuticle advances the loss of cuticle. S100A3 protein, located at the endocuticle, was found to be citrullinated and transformed into tetramer to improve its Ca2+‐binding ability. It is postulated that this biochemical property affects the maturation of cuticle and contributes to its reinforcement.AimsThis study aims to elucidate the role that S100A3 plays in age‐dependent cuticle damage.MethodsHair fibers collected from Japanese females were evaluated for the content and citrullination rate of S100A3, incidence of type E damage, and resistance of cuticle.ResultsIn the aged hair, the content of S100A3 was positively correlated with the level of type E damage and low resistance to stress. Hair fibers in which S100A3 is highly citrullinated, however, showed low levels of type E damage and high resistance of cuticle, even in the aged hair as well as at younger ages.ConclusionsS100A3 and its citrullination process are related to rigidity of endocuticle of aged hair.