Most patients with polycythemia vera and half with idiopathic myelofibrosis and essential thrombocythemia have an acquired V617F mutation in JAK2. Using sensitive polymerase chain reaction (PCR)-based methods, we genotyped 152 patients with idiopathic myelofibrosis to establish whether there were differences in presentation and outcome between those with and those without the mutation. Patients positive for V617F had higher neutrophil and white cell counts (P = .02) than did patients negative for V617F, but other diagnostic features were comparable between the 2 groups. Patients positive for V617F were less likely to require blood transfusion during follow-up (P = .03). Despite this, patients positive for V617F had poorer overall survival, even after correction for confounding factors (P = .01).