Early phases of polycythemia vera, essential thrombocythemia, and primary myelofibrosis (PMF) can be difficult to distinguish by morphologic studies alone because they share many morphologic characteristics. Histologic criteria according to the 2008 World Health Organization (WHO) classification are part of the myeloproliferative neoplasia (MPN) diagnosis. Our aim was to assess the reproducibility of morphologic characteristics and determine their relative importance for histologic diagnoses on selected trephine biopsy sections. For the study, 56 prefibrotic MPN trephine specimens were blindly reviewed by 4 hematopathologists using a scoring list of 16 histologic characteristics mentioned in the WHO classification. Consensus was defined as agreement by 3 of 4 hematopathologists. High degrees of consensus were reached for individual major morphologic features used in the WHO classification, especially for the nuclear features of megakaryocytes (83%). Some of the features correlated positively or negatively with the histologic diagnosis of PMF. Consensus for the histologic classification of MPN was reached in 39 (70%) of 56 cases without knowledge of clinical data. This finding indicates a difference in the relative importance assigned to individual histologic characteristics by different hematopathologists.