Background: Myeloproliferative neoplasms likely involve both myeloid and lymphoid lineages. Nevertheless, the coincidence of chronic myeloproliferative and lymphoproliferative diseases in the same patient is a rare phenomenon. Methods: We report a case of a patient having essential thrombocythemia (ET) and B-chronic lymphocytic leukemia (B-CLL). In this patient and in 2 relatives with lymphoproliferative disorders, we searched for JAK2(V617F) mutation in lymphocytes. Results: In the patient with ET and B-CLL, we identified homozygous JAK2(V617F) mutation in the granulocytic compartment. Both relatives were heterozygous for JAK2(V617F) mutation, whereas no mutation signal could be detected in the lymphoid compartment of all 3 patients. Conclusion: Our results seem to confirm that CLL cases are negative for JAK2(V617F) mutation in B- and T-lymphocyte populations. Presence of JAK2(V617F) mutation in subjects without myeloproliferative diseases could indicate an increased risk of a future myeloproliferative neoplasm development.