Impaired immune reconstitution has moved to the forefront of clinical problems limiting progress in allogeneic bone marrow transplantation (BMT). The identification of therapies that can enhance immune reconstitution by increasing thymopoiesis is critical to solving this problem. Interleukin 7 (IL-7) is the most potent thymopoietic cytokine identified thus far. To study the effects of IL-7 on immune reconstitution and graft-versus-host disease (GVHD) following allogeneic BMT, we administered recombinant human IL-7 (rhIL-7) in a murine parent into an F1 model. Results showed that rhIL-7 therapy lowered the "threshold" T-cell dose required to induce both clinical signs of GVHD as well as lethal GVHD. Histologic analysis of GVHD target tissues revealed that rhIL-7 increased the degree of inflammation and tissue damage observed at all T-cell doses studied, but did not change the pattern of organs affected or the histologic appearance of the GVHD within target organs. In addition, we evaluated the capacity for rhIL-7 to enhance thymopoiesis in the setting of allogeneic T cell-depleted (TCD) and T-cell-replete BMT. We observed that rhIL-7 therapy enhanced thymic function in TCD allogeneic BM transplant recipients, but not in animals that received even modest doses of T cells presumably due to thymic toxicity of the graft-versus-host reaction. Thus, caution must be exercised as IL-7 is developed clinically as an immunorestorative agent for use in the setting of allogeneic BMT. These results suggest that use of IL-7 should be limited to the setting of TCD BMT to obtain the greatest benefit on immune competence with the least toxicity.