New automated hematology analyzers have led to the availability of novel hematological parameters, including the immature platelet fraction (IPF) and the immature reticulocyte fraction (IRF), both of potential interest in patients with myeloproliferative neoplasms (MPNs). We performed a prospective analysis of 217 patients with MPN, including 32 (15%) with essential thrombocythemia (ET), 43 (20%) with polycythemia vera (PV), and 142 (65%) with myelofibrosis (MF); the IPF and IRF were measured by the Sysmex XN analyzer. As compared to patients with ET, both a higher IPF and IRF were observed among patients with PV and MF. Factors associated with high IPF among patients with PV/ET were male sex, thrombocytopenia, and diagnosis of PV; among patients with MF, they were elevated peripheral blasts, low platelet count, JAK2 V617F mutation, and previous therapy. Factors associated with high IRF among patients with PV/ET were low hemoglobin, high reticulocyte count, and PV diagnosis; among patients with MF, they were peripheral blasts and elevated reticulocytes. The IPF and IRF represent novel parameters in patients with MPN with potential relevant clinical implications. Comparison with healthy subjects and those with secondary polycythemia is needed to confirm our preliminary findings.