Polycythemia vera, essential thrombocytosis, and myelofibrosis are chronic Philadelphia-negative myeloproliferative neoplasms that are characterized by clonal hematopoiesis, splenomegaly, risk of hemorrhagic and thrombotic sequelae, and profound symptom burden. We review the outcomes of 75 myeloproliferative neoplasm patients treated with pegylated interferon alpha 2a off study at an academic medical center. In the 56 treated polycythemia vera and essential thrombocytosis patients, a complete or partial response was obtained in 78.6% of patients per ELN/IWG-MRT revised criteria, with >80% of polycythemia vera patients becoming phlebotomy independent and 60% of essential thrombocytosis patients having platelet normalization with therapy. In the 19 treated myelofibrosis patients, stable disease was seen in 63.2% of patients. Vascular events occurred in 2/75 (2.6%) of treated patients while on therapy. Grade 3 toxicity was uncommon with leukopenia noted in 1 patient (1.3%). The most common adverse event overall was grade 1 fatigue in 18.7%. This retrospective single center analysis demonstrates pegylated interferon alpha 2a is active and well-tolerated therapy outside the support of a clinical trial. These results substantiate the previously reported efficacy of pegylated interferon alpha 2a in myeloproliferative neoplasms. Further prospective and randomized clinical trial data is required to better delineate pegylated interferon alpha 2a's use in myeloproliferative disease, with emphasis placed on comprehensive molecular characterization, allelic burden quantification, and measurement of histologic response. (C) 2017 Elsevier Ltd. All rights reserved.