More than a decade has passed since the publication of the first report implicating T cells in the bone loss induced by ovariectomy (ovx).(1) Since then there has been extraordinary progress in the understanding of the regulatory network that links the hemopoietic and the mesenchymal compartments of the bone marrow (BM), the interactions between the immune system and bone, and the role of lymphocytes as mediators of the effects of calciotrophic hormones in bone. Collectively this body of knowledge has led to the firm establishment of ‘‘osteoimmunology’’as a novel discipline and a promising area of investigation. The objective of this perspective is to revisit the exciting hypothesis that T cells play a pivotal role in the mechanism of ovx-induced bone loss.