The association between breast cancer and glioblastoma multiforme has not been amply analyzed in the literature. We describe 11 female patients with a diagnosis of glioblastoma multiforme who were treated when younger for breast cancer. We believe that this association is not due to chance but rather to genetic changes in hormone status and in particular to sex hormones. Another important point of view is represented by the chemotherapy treatment of breast cancer, which could have a carcinogenic effect and explain the growth of glioblastoma. This consideration, in our opinion, is important, because more effort should be made to understand the pathogenesis of glioblastoma multiforme and to improve the therapeutic approaches.