Primary chronic myelofibrosis (PCMF) is a rare myeloproliferative disorder among young individuals. Conventional therapies are often ineffective and only palliative. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) using myeloablative or reduced-intensity conditioning offers the only possible curative treatment. However, the course of PCMF is highly variable from only a few months to more than a decade. To identify PCMF patients for allo-HSCT candidates, we analyzed a Japanese cohort aged less than 70 years and established a new prognostic system based on simple clinical parameters. In a training series, we used 207 patients with complete data, whose diagnoses were made over a period of 10 years (1988 to 1997) (Okamura T et al. Int J Hematol 2001). Patients who received allo-HSCT were not included. Using the Cox proportional hazard regression model, four initial variables were independently associated with shorter survival: Male sex (hazard ratio [HR], 2.26; 95% confidence interval [C.I.], 1.35–3.46), Hemoglobin < 10 g/dL (HR, 3.01; 95% C.I., 1.83–4.93), the presence of constitutional symptoms (fever, sweats and weight loss) (HR, 2.33; 95% C.I., 1.33–4.06), and circulating blasts ≥ 1% (HR, 1.53; 95% C.I., 1.00–2.33). Based on the above criteria, two groups were identified; a low-risk goup with up to one adverse prognostic factor and a high-risk group with two or more adverse prognostic factors. With the median follow-up time of 83 months, the median survival was 292 months in the low-risk group, and 66 months in the high-risk group, respectively (P <.0001, Figure). An external data set of 100 patients who diagnosed after 1999 was used for validation. The 4 prognostic systems separated the validation series in groups with significantly different survival rates despite the median follow-up time of 12 months. A simple prognostic model using the combination of sex, Hb level, constitutional symptoms and circulating blasts was developed and validated. This model might help to select patients with PCMF for treatment and to evaluate therapeutic results including allo-HSCT.