The bone marrow biopsy in mastocytosis often reveals characteristic mast cell collections associated with lymphoid cell aggregates. There is limited information on the composition of these aggregates and whether they contain cytokines important to mast cell growth and development. We thus performed an immunohistochemical characterization of bone marrow lesions in 7 patients with systemic indolent mastocytosis. In nodular lesions, the collections of mast cells were surrounded by lymphoid aggregates; these aggregates consisted of a mixture of B and T cells. Immunohistochemical staining for stem cell factor (SCF) revealed the presence of this cytokine in the lesional mast cells with a granular staining pattern. These results show that the characteristic nodular lesions observed in bone marrow biopsy specimens of patients with mastocytosis bearfeatures of benign lymphoid aggregates and that SCF is present in these lesions, creating a potential autocrine or paracrine growth and differentiation loop for mast cells and lymphoid progenitors.