<p><b>OBJECTIVE</b>To compare the morphologic features of bone marrow (BM) between the prefibrotic-early primary myelofibrosis (PMF) and essential thrombocythaemia (ET).</p><p><b>METHOD</b>Seven cases of prefibrotic-early PMF were selected and analyzed. Based on the diagnostic standard of prefibrotic-early PMF by WHO, BM aspirate smears, trephine biopsy sections and imprints of 156 uncertain ET cases conducted simultaneously were recruited into this study, the BM morphologic features between the prefibrotic-early PMF and ET groups were analyzed. The morphological difference in 22 cases of prefibrotic-early PMF and 27 ET were compared between the JAK2V617F mutation positive and negative groups.</p><p><b>RESULTS</b>Of the 156 uncertain ET cases, it was reclassified 61 prefibrotic-early PMF (34 MF-0, 27 MF-1), 12 PMF and 83 ET. The platelet count and LDH level in MF-1 group were obviously higher than that of ET group (P < 0.05). The blast percentage of BM smear in MF-1 group was also higher than that of ET group (P < 0.05). As to BM section, cases with increased nucleated cells (granulocyte), compact megakaryocytic cluster, megakaryocyte near bone trabecula, cloud-like megakaryocyte, small bare nucleus of megakaryocyte and large ball-like megakaryocyte in MF-0 and MF-1 group were significantly higher than that of ET group (all P < 0.05), cases with megakaryocytic cluster of various size in MF-1 group were significantly higher than that of MF-0 and ET groups (P < 0.05). The JAK2V617F mutation rate in prefibrotic-early PMF and ET groups were 54.5% and 48.1%, respectively. Hb level in JAK2V617F mutation positive group was obviously higher than the negative group (P < 0.05), no special change with megakaryocytic morphology was found between the positive and negative groups.</p><p><b>CONCLUSION</b>Morphology of BM section, especially megakaryocytic morphologic characteristics are the main basis in distinguishing prefibrotic-early PMF from ET. The importance of morphologic index were megakaryocytic cluster with various size, cloud-like megakaryocyte, large ball-like megakaryocyte, increased nucleated cells (granulocyte), small bare nucleus, megakaryocyte near bone trabecula and compact megakaryocytic cluster in order. JAK2V617F mutation provides no specific effect on the megakaryocytic morphology.</p>