The association of chronic lymphocytic leukemia (CLL) with essential thrombocythemia (ET) is an extremely rare event and until now 3 patients with such coexistence have been reported in the literature. We report a 77-year-old white woman in whom these two disorders were diagnosed concomitantly on the basis of peripheral blood count and cytology, bone marrow cytology and histology, immunophenotyping, as well as exclusion criteria. The diagnosis of ET was also supported by spontaneous in-vitro erythroid colony growth and by evaluation of thrombopoietin (TPO) serum level. Interphase FISH analysis allowed to detect 13q14.3 deletion in 98% of lymphocytes nuclei. In contrast this aberration was not observed in the megakaryocytes. The results of PCR analysis of IgG gene rearrangement showed distinct bands characteristic for monoclonal lymphoid population in bone marrow, peripheral blood and inguinal lymph node. The patient was started on hydroxyurea 1 g/day and normalization of the platelet count was achieved. Possible etiopathogenic relationships between both disorders and differential diagnosis of ET and reactive thrombocytosis (RT) are discussed.