Parathyroid hormone (PTH) promotes bone catabolism by targeting bone marrow (BM) stromal cells (SCs) and their osteoblastic progeny. Here we show that a continuous infusion of PTH that mimics hyperparathyroidism fails to induce osteoclast formation, bone resorption, and cortical bone loss in mice lacking T cells. T cells provide proliferative and survival cues to SCs and sensitize SCs to PTH through CD40 ligand (CD40L), a surface molecule of activated T cells that induces CD40 signaling in SCs. As a result, deletion of T cells or T cell-expressed CD40L blunts the bone catabolic activity of PTH by decreasing bone marrow SC number, the receptor activator of nuclear factor-κB ligand (RANKL)/OSTEOPROTEGERN (OPG) ratio, and osteoclastogenic activity. Therefore, T cells play an essential permissive role in hyperparathyroidism as they influence SC proliferation, life span, and function through CD40L. T cell-SC crosstalk pathways may thus provide pharmacological targets for PTH-induced bone disease.