Hyperostosis frontalis interna is a restricted bilateral thickening of the frontal endocranial surface, which is frequently found in postmenopausal females today. Surprisingly, this condition had a higher male prevalence in its rare archaeological records. This is again highlighted by the oldest known male European hyperostosis frontalis interna case in an adult Celtic from 100 BC presented here. This unique specimen supports earlier suggestions of the possible microevolution of human endocrine regulation, e.g. by sex steroids, and its pathoanatomical impact.