In this study, the investigators will determine the utility of Tamoxifen, a non-cytotoxic agent, to improve quality of life, biochemical parameters, and bone marrow involvement in systemic mastocytosis patients having 1) up to 20% bone marrow infiltration by mast cells and/or 2) mediator-release symptoms which are not controlled by tolerated doses of standard "non-cytotoxic" medications regardless of the percentage bone marrow involvement by mastocytosis. The dose of Tamoxifen will be 20 mg/day and the duration of treatment will be for one year. Patients currently taking interferon alfa, imatinib mesylate, or cladribine will be excluded until these medications have been stopped.