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.

Not desired

Inclusion Criteria:

• Systemic Mastocytosis

Exclusion Criteria:

• Current treatment with Imatinib mesylate, cladribine or interferon alpha.

Patients with systemic mastocytosis having up to 20% bone marrow involvement or clinical symptoms not controlled on current medications.