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 40% 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 40 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.