A 63-year-old woman was referred to our department in 2015 because of anemia and thrombocytosis. MPL W515/K was positive, JAK-2V617F and CALR exon 9 were negative. Bone marrow(BM)biopsy led to a diagnosis of primary myelofibrosis (PMF)in the prefibrotic/early stage(Grade 1). BMbiopsy performed in 2016 showed overt fibrotic stage(Grade 2). She was classified according to the Dynamic International Prognostic Scoring System(DIPSS)as intermediate(Int)-Ⅱrisk. Ruxolitinib 10 mg daily was initiated. Ruxolitinib was suspended for hepatic dysfunction after the dose was increased to 15 mg. Subsequently, ruxolitinib was resumed at 10 mg. BM biopsy performed in 2017 showed progression of myelofibrosis(MF)to Grade 3. BM biopsy performed in 2018 showed improved to Grade 0-1, however, BM was fatty. Currently in 2019, she continues to be on ruxolitinib. Results of immunohistochemical staining of BM biopsy specimens for cytokines and CD34 suggested the role of cytokines in the pathogenesis of the PMF. It was speculated that ruxolitinib blocked the production of cytokines to ameliorate the MF and restore the hematopoietic function of the BM. Although the pathogenesis of the fatty marrow remained unclear, the possibility of involvement of ruxolitinib cannot be denied.