Recent reports have described complete or major molecular remission in patients with polycythemia vera after long-term treatment with the immunomodulatory agent IFN-a2. Accordingly, there are reasons to believe that the immune system is a key player in eradicating the JAK2 mutated clone in these patients. Foxp3(+) regulatory T cells play a pivotal role in maintaining immune homeostasis and, importantly, preventing immune reactivity to self-antigens; however, their suppressive activity can compromise an effective antitumor immune response, and high frequencies of regulatory T cells in peripheral blood have been reported in both hematologic and solid cancers. We have analyzed the number, phenotype, and function of circulating CD4(+)CD25(+)Foxp3(+) T cells in patients with chronic myeloproliferative neoplasms. Surprisingly, we found a marked expansion of this subset of lymphocytes in patients treated with IFN-a2 (13.0%; 95% confidence interval [CI] 10.8% to 15.2%) compared with healthy donors (6.1%; 95% CI 4.9% to 7.2%), patients with untreated chronic myeloproliferative neoplasms (6.9%; 95% CI 5.8% to 7.4%), or patients treated with hydroxyurea (5.8%; 95% CI 4.3% to 7.4%; P <.0001).