A retrospective study of 38 essential thrombocythemia (ET) patients was conducted, reviewing bone marrow biopsies according to WHO criteria using a semiquantitative scoring system. Four patients did not fulfil the WHO criteria for a myeloproliferative disorder and one biopsy was insufficient for evaluation. 14 patients were reclassified as having prefibrotic idiopathic myelofibrosis (IMF), whilst the ET diagnosis was sustained in 19 patients. The individual bone marrow parameters of the reviewed diagnosis showed no correlation with JAK2 V617F mutation status, which was determined by a highly sensitive quantitative real-time PCR (qPCR) method. However, we could confirm previous findings of higher haemoglobin and lower platelet levels in the JAK2 V617F positive patients. Thus, the well-established phenotypic relationship of JAK2-positive ET and PV at the biochemical and molecular level was not recorded as regards bone marrow morphology according to the WHO criteria. Accordingly, the WHO concept of two distinct entities, ET and prefibrotic IMF, does not seem to fit the model of JAK2-positive ET as part of a biological continuum of JAK2 V617F-positive chronic myeloproliferative disorders.