The purpose of this study is to assess the real-world safety of fedratinib for the treatment of adult participants with primary myelofibrosis (PMF), post polycythemia vera myelofibrosis (post-PV MF), or post essential thrombocythemia myelofibrosis (post-ET MF) who were previously treated with ruxolitinib. Participants will represent the overall patient population with PMF, post-PV MF or post-ET MF who lost adequate response to and/or are intolerant to ruxolitinib. Inadequate response definitions will follow Ministry of Food and Drug Safety-approved label and reimbursement criteria of the Health Insurance Review \& Assessment Service.