Pegylated interferon alpha-2a (Peg-IFN) has been shown to induce hematologic and molecular responses in patients with the Philadelphia-negative myeloproliferative neoplasms (MPNs), including polycythemia vera (PV) and essential thrombocythemia (ET). We describe a series of patients with long-standing MPNs among whom Peg-IFN was initiated when they developed anemia and increased bone marrow reticulin fibrosis suggestive of early transformation to post-ET (PET) or post-PV (PPV) myelofibrosis (MF). Six patients were treated with Peg-IFN for a mean duration of 33.8 months (range 2-63 months). Five patients had long-standing ET (three were calreticulin (CALR)-positive, one janus kinase 2 (JAK2)-positive, and one JAK2-negative and CALR-negative), and one had long-standing JAK2-positive PV prior to starting Peg-IFN. This is the first study to report that, concurrent with the improvement in anemia, serial laboratory studies demonstrate an increase in serum LDH and left-shifted myeloid cells in the peripheral circulation over approximately 6 months, followed by a gradual normalization of these findings. Splenomegaly also increased and then resolved among responding patients. Serial bone marrow biopsies were available, which showed little change except for improvement in the grade of reticulin fibrosis in two patients. Among patients with early transformation to PET or PPV MF, our data support the efficacy of Peg-IFN in improving hemoglobin levels and reducing splenomegaly. These peripheral blood findings should not, therefore, be considered evidence of treatment failure within the first year of Peg-IFN therapy.