Cytoreductive therapy in systemic mastocytosis (SM) includes several drugs whose individual merit has not been well characterized. We retrospectively studied 108 Mayo Clinic patients who met the 2008 WHO diagnostic criteria for SM and received at least one cytoreductive drug. The numbers of patients who were evaluable for response to treatment with interferon-alpha with or without prednisone (IFN-α), hydroxyurea (HU), imatinib mesylate (IM) or 2-chlorodeoxyadenosine (2-CdA) were 40, 26, 22, and 22, respectively. The corresponding overall (major) response rates, according to recently published consensus criteria, were 53% (18%), 19% (0%), 18% (9%), and 55% (37%). The respective overall response rates in indolent SM, aggressive SM and SM associated with another clonal hematological nonmast cell lineage disease (SM-AHNMD) were 60%, 60%, 45% for IFN-α, 0, 0, 21% for HU, 14%, 50%, 9% for IM and 56%, 50%, 55% for 2-CdA. The absence of mast cell mediator release symptoms in IFN-α-treated patients and presence of circulating immature myeloid cells in 2-CdA-treated patients predicted inferior response. TET2 mutational status did not influence treatment response. Although the major response rates with these four cytoreductive agents were still suboptimal and HU was mainly used in patients with SM-AHNMD, the current study favors 2-CdA or IFN-α as first-line current therapy in SM and identifies patients who are likely to respond to such therapy. Am. J. Hematol., 2009. © 2009 Wiley-Liss, Inc.