Polycythemia vera and primary myelofibrosis share a propensity to progress toward a myelofibrotic late stage with overlapping clinical characteristics. Bone marrow features potentially useful for distinguishing the two entities have not been thoroughly investigated and, currently, clinical history is used for purposes of disease classification. This study describes in detail the morphologic features of 23 cases of post-polycythemic myelofibrosis and 15 cases of primary myelofibrosis with a similar degree of fibrosis, from two large medical centers. Cytogenetic results were available in 19 post-polycythemic myelofibrosis and in 13 primary myelofibrosis cases. JAK2 status and follow-up information was available in all cases. Cellularity was increased in both groups, but more so in post-polycythemic myelofibrosis than in primary myelofibrosis. In post-polycythemic myelofibrosis, most megakaryocytes retained polycythemia vera-like features including normally folded and/or hyperlobulated nuclei devoid of severe maturation defects; only in a few cases were rare tight clusters present. In primary myelofibrosis cases, megakaryocytes showed pronounced anomalies, including increased nuclear:cytoplasmic ratio, abnormal clumping of chromatin and frequent tight clustering. No differences in blast number (<1%) or in the myeloid:erythroid ratio were observed. Post-polycythemic myelofibrosis showed a higher degree of karyotypic alterations and higher percentage of cases with complex karyotype and/or two or more clones. Chromosome 1 defects were common in post-polycythemic myelofibrosis, whereas isolated del(20q) was the most common alteration in primary myelofibrosis. No survival differences were noted between the two groups. Post-polycythemic myelofibrosis cases retain a distinct megakaryocytic morphology that represents a useful clue for differential diagnosis. In addition, they more often display a complex karyotype than do primary myelofibrosis cases. These results suggest that myelofibrosis in polycythemia vera represents a form of progression characterized by profound genetic damage whereas in primary myelofibrosis it is an intrinsic part of the phenotypic manifestation of the disease, not necessarily associated with adverse cytogenetics.