To contribute to a better knowledge of the prognosis of idiopathic myelofibrosis (IM), the prognostic value of the presenting features in 106 patients diagnosed with IM at a single institution during a 21‐year period was retrospectively analysed. Median survival was 59.4 months (95% CI 40.7–75.4). Using univariate analysis, age < 64 years, constitutional symptoms (fever, night sweats, weight loss), Hb < 10 g/dl, circulating blasts (≫1%), and serum LDH > 3 times upper normal level were associated with a significantly shorter survival; male sex, platelet count <100×10⁹/l, blood percentage of immature granulocytes (excluding blasts), low cholesterol levels and advanced marrow histological stage had borderline significance. Using multivariate study, only age > 64 years, constitutional symptoms, Hb < 10 g/dl, and circulating blasts retained their prognostic relevance. The latter three variables confirmed their predictive value in patients above and below the series median age, and were able to identify two groups of patients: a low‐risk group of 67 patients with none or one bad prognostic factor, in whom IM had an indolent course (median survival 98.8 months, 95% CI 68.7–127.6), and a high‐risk group, including 39 patients with two or three factors, with a more aggressive disease (median survival 20.6 months, 95% CI 10–28.2). Finally, the application of two recently proposed scoring systems (in which three prognostic groups are considered) was unable to separate intermediate‐ from high‐risk patients.