SUMMARY Myelofibrosis is characterized by progressive splenomegaly, cytopenias and debilitating constitutional symptoms. It has the worst prognosis and poorest quality of life of all the chronic myeloproliferative neoplasms. Allogeneic stem cell transplantation is the only curative therapy, but it carries high treatment-related risks and is thus available to only a small subset of patients. All other interventions merely palliate either anemia or splenomegaly. Ruxolitinib, a JAK1/2 inhibitor, has recently been shown to be effective in reducing splenomegaly and improving constitutional symptoms to a degree that has not been achieved with conventional therapy. However, treatment with ruxolitinib can often worsen anemia, and its ability to change the natural history of myelofibrosis has not been definitively established.