Spleen enlargement, present in 10–20% of Essential Thrombocythemia (ET) patients at diagnosis, is a feature clinically easy to assess, confirmable by echography with a very low chance of misinterpretation. Nonetheless, the clinical and prognostic role of splenomegaly has been seldom evaluated. From 1979 to 2013, 1297 ET patients retrospectively collected in the database of the Lazio Cooperative Group and Bologna University Hospital were evaluable for spleen enlargement at diagnosis and included in the analysis. On the whole, spleen was enlarged in 172/1297 (13.0%) patients; in most cases (94.8%) splenomegaly was mild (≤5 cm). Patients with splenomegaly were younger, predominantly male, presented higher platelet count and JAK2V617F allele burden and had a lower incidence of concomitant cardiovascular risk factors. At least one thrombotic event during follow-up occurred in 97/1,125 (8.6%) patients without spleen enlargement compared to 27/172 (15.7%) patients with spleen enlargement (P = 0.003). Despite comparable use of cytoreductive/antiplatelet therapies in the two groups, the cumulative risk of thrombosis at 5 years was significantly higher in patients with baseline splenomegaly (9.8% versus 4.4% in patients without splenomegaly, P = 0.012). In multivariate analysis exploring risk factors for thrombosis, splenomegaly retained its negative prognostic role, together with previous thrombosis, leucocyte count and male gender. Baseline splenomegaly seems to be an independent additional risk factor for thrombosis in nonstrictly WHO-defined ET patients. This data could be useful in the real-life clinical management of these patients. Am. J. Hematol. 91:318–321, 2016. © 2016 Wiley Periodicals, Inc.