Until today, JAK2 alterations have been mainly associated with myeloid malignancies among which they play a key pathogenic role in chronic myeloproliferative neoplasms. More recently, aberrations involving the JAK2 gene have also been reported in lymphoid diseases, including acute leukemia and lymphomas. In addition, the constitutively activating JAK2V617F mutation has been identified in some patients affected by B-chronic lymphocytic leukemia with a concomitant myeloproliferative neoplasm. Interestingly, these cases could help us to better understand the pathogenesis of these myeloid and lymphoid diseases and reveal if they share a common ancestral progenitor or just coincide. The involvement of JAK2 in lymphoid neoplasms may suggest the possibility of new therapeutic approaches broadening the use of JAK1-2 inhibitors also to these malignancies.