Myelofibrosis (MF) may develop in all types of myeloproliferative disorders and its identification is of clinical relevance. Typical bone marrow (BM) morphology of patients with essential thrombocythemia (ET) shows either "normal" amount or "a slight increase" of reticulin fibers, but the published data differ in relation to the applied MF definition and ET diagnostic criterias. The aim of this study was to evaluate retrospectivelly MF in BM biopsies of 30 cases in which the diagnosis of ET was confirmed also clinically by local hematologists. In 7 of the patients not only primary but also sequential biopsy was available. The MF grade and extent were evaluated semiquantitativelly in archival slides stained by Gömöri silver impregnation. The analysis was based on the European clinicopathological criteria 2004 (ECP) defining a) normal bone marrow fibrosis (MF0), b) slight reticulin fibrosis (MF1), c) advanced reticulin and initial collagen fibrosis (MF2) and d) advanced collagen fibrosis (MF3). Generally, in majority of the biopsies MF0 (n = 6) or MF1 (n = 25, 18x focal and 7x diffuse) was found. More advanced MF2 was much less common as it was present in 6 biopsies (5x focal and 1x diffuse). In relation to the actual time of BM biopsy during course of the disease, the introductory biopsies done at the time of diagnosis (n = 18) showed 3x MF0, 14x MF1 and 1x MF2. The biopsies performed after a long time of patients observations (n = 12) showed 3x MF0, 7x MF1 and 2x MF2. In 5 of 7 sequential biopsies the progress of MF was evident, but 4 of these patients were treated by cytoreductive therapy. We conclude that the BM of patients with ET in initial phase shows either MF0 or focal slight increase of reticulin fibers (MF1). In addition, the long course of the disease and/or applied therapy may lead to more developed MF and more advanced MF stages (diffuse MF1 or MF2). Therefore their finding in the BM biopsies examined in the later phases of the disease should not exclude the diagnosis of ET.