© 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.In some patients with chronic myeloproliferative neoplasms, myelofibrosis (MF) develops as natural evolution of the disease. The aim of this study was to analyze predictive factors that may cause MF in polycythemia vera (PV) and essential thrombocythemia (ET) patients. This retrospective study was conducted on PV and ET patients who attended our hospital between 2008 and 2019. The development of MF during follow-up was recorded, and comparisons were made of the patients who developed MF and those who did not develop MF. Evaluation was made of 126 ET and 105 PV patients. During follow-up period, MF had developed 5.7% of PV and 7.1% of ET patients. It was determined that JAK-2 mutant allele burden, lymphocyte count, vitaminB12 levels, and grade of bone marrow fibrosis at diagnosis had statistically significant impact on the development of MF in all patients. In the logistic-regression analysis, it was found that initial hemoglobin, hematocrit, neutrophil-to-lymphocyte ratio, and monocyte count for PV patients; vitaminB12, the presence of splenomegaly; and BM fibrosis at diagnosis for ET patients have statistically significant effect on MF development. The results of the current study demonstrated that some parameters especially vitamin B12 levels can be used as predictive markers for the development of MF.