BACKGROUND The diagnosis of myeloproliferative disorders (MPDs) and myelodysplastic syndromes (MDSs) has historically relied on combining clinical information with the morphologic features of the peripheral blood and bone marrow to reach a final diagnosis. Objective evidence of a myeloid stem cell neoplasm in the form of a clonal cytogenetic abnormality is provided in only 30% to 40% of the non-chronic myeloid leukemia (CML) chronic MPDs (non-CML MPDs) and in a similar percentage of the MDSs. OBJECTIVE To identify normal patterns of antigen expression during myeloid maturation and to determine whether flow cytometric evaluation of myeloid maturation represents an additional objective way to assess the likelihood of a stem cell neoplasm. METHODS We retrospectively evaluated 4-color flow cytometry data from more than 400 bone marrow aspirates obtained since 1998 from patients suspected of having a non-CML MPD or an MDS. RESULTS Reproducible patterns of antigen expression were seen in normal myeloid maturation as well as in benign reactive settings such as marrow regeneration. In addition, we summarize data, presented in detail elsewhere, from a retrospective comparison of the sensitivity of flow cytometry with conventional cytogenetics for a large number of bone marrow aspirates on which both types of studies were performed. These data indicate that more than 90% of non-CML MPD and MDS cases with a clonal cytogenetic abnormality will be identified as abnormal by 4-color flow cytometry, and they therefore validate the use of flow cytometry in the diagnosis of these disorders. CONCLUSIONS In experienced laboratories, 4-color flow cytometry represents a valuable addition to the workup of non-CML MPDs and MDSs.