In this protocol, we propose to treat patients with advanced forms of mastocytosis with an allogeneic stem cell transplant from an HLA-identical sibling, using a low intensity non-myeloablative regimen. This approach has the advantage of decreasing the transplant-related toxicity while allowing adequate immunosuppression to establish stem cell and lymphocyte engraftment. Donor derived CD4 and CD8 lymphocytes, which are important in killing of leukemic cells by mounting a "graft versus leukemia" effect, should be useful in the elimination of aberrant mast cells and their progenitors, that is "graft-versus-mast cell effect". This mechanism may be particularly relevant in mastocytosis as point mutations of c-kit may constitute an antigenically distinct T-cell target for recognition by the engrafted donor cells.