Primary myelofibrosis (PMF) is a myeloproliferative neoplasm that may display a variable degree of cytopenia and dysplasia sometimes difficult to distinguish from myelodysplastic syndrome with myelofibrosis (MDS-MF). We reviewed flow cytometric features of bone marrow from 70 cases of PMF and compared them with those from 17 cases of MDS-MF and 20 nonneoplastic control cases. The results were correlated with JAK2(V617F) and cytogenetic findings. Granulocytes and monocytes from PMF cases exhibited multiple dysplastic features overlapping with those of MDS-MF at a comparable or higher frequency: low side scattering, aberrant CD56 expression in granulocytes and monocytes, and an abnormal CD13/CD16 maturation pattern. Unique to PMF was the small granulocyte size compared with that of MDS-MF and control cases. Although the percentage of CD56+ granulocytes and monocytes did not correlate with JAK2(V617F) or cytogenetic abnormalities, a subset analysis of 36 cases revealed that median fluorescence intensity of CD56 expression correlated positively with the presence of cytogenetic abnormalities. Our findings indicate that although there is considerable overlap between PMF and MDS-MF, the smaller granulocytes observed in PMF are a useful distinguishing feature.