We report that osteopenia is a prominent and previously unappreciated clinical feature of patients with X-linked hyper-IgM syndrome, an inherited immune deficiency disorder caused by mutations in the gene encoding CD40 ligand (CD40L). We therefore conducted studies to determine the relationship between CD40L and osteoclastogenesis. Recognizing that activated T cells express surface receptor activator of NF-kappaB ligand (RANKL) and can induce osteoclast differentiation of myeloid cells expressing RANK, we assessed the capacity of wild-type T cells and CD40L(-/-) T cells to induce osteoclastogenesis in vitro. Relative to wild-type T cells, activated CD40L(-/-) T cells from both humans and mice promoted robust osteoclast differentiation of myeloid cells. Whereas activated CD40L(-/-) T cells had normal expression of RANKL, they were deficient in IFN-gamma production. In subsequent studies, we cultured activated CD40L(-/-) T cells in the presence of IFN-gamma, and we found that the osteoclastic capacity of CD40L(-/-) T cells could be greatly diminished. These results show that CD40L can influence RANKL signaling through T cell priming, and thus they demonstrate a regulatory role for CD40L in bone mineralization that is absent in patients with X-linked hyper-IgM syndrome.