Primary myelofibrosis (PMF) is a Philadelphia chromosome-negative chronic myeloproliferative neoplasm usually affecting elderly people. Median survival currently approaches 6 years, with a few patients surviving more than 20 years but others dying soon after diagnosis. In 10% to 20% of patients, PMF evolves into acute leukemia. Conventional treatment is merely palliative and does not prolong survival. The only chance for cure is allogeneic hemopoietic stem-cell transplantation (allo-HSCT), which is associated with high morbidity and mortality. Introduction of less aggressive forms of allo-HSCT and the prospect of molecular-targeted therapies has renewed interest in prognostication in PMF. The most important prognostic factors are anemia, age over 65 years, constitutional symptoms, leukocytosis, blood blasts, and some cytogenetic abnormalities. These factors have recently been systematized into an International Prognostic Scoring System (IPSS) and further refined in a dynamic IPSS (DIPSS), useful at any time over the disease course, as well as a DIPSS-plus model including karyotypic information.