Mutations in the calreticulin (CALR) gene were recently discovered in patients with essential thrombocythemia (ET) lacking the JAK2V617F and MPLW515 mutations, but no information is available on the clinical correlates. In this series, CALR mutations were found in 15.5% of 576 WHO-defined ET subjects accounting for 48.9% of JAK2 and MPL wild type. CALR mutated patients were preferentially male and showed higher platelet count, lower hemoglobin and leukocyte count compared with JAK2 and MPL mutated patients. Subjects carrying CALR mutation had a lower risk of thrombosis than JAK2 and MPL mutated; of interest, their risk was superimposable to patients who were wild type for the above mutations. CALR mutation had no impact on survival or transformation to post-ET myelofibrosis. Genotyping for CALR mutations represents a novel useful tool for establishing a clonal myeloproliferative disorder in JAK2 and MPL wild type patients with thrombocytosis and may have prognostic and therapeutic relevance.