The automated analysis of bone marrow aspirates was performed to estimate the bone marrow cellularity and differential cell count. Total nucleated cell count (TNC) was measured using an automated hematology analyzer. TNC of the bone marrow correlated well with the bone marrow cellularity estimated by microscopic examination (r = 0.590, p <0.001). The bone marrow cellularity was readily confirmed as <30%, from 30 to 70%, or >70% according to TNC. Differential count of bone marrow cells was done with the combination of CD45 monoclonal antibody and propidium iodide using flow cytometry. Excellent correlations were obtained for the population distributions of normoblasts, eosinophils, lymphocytes, and blasts between the results of flow cytometry and manual differential counts. The percentage of mature granulocytes in flow cytometry showed good correlation with the manual percentage of metamyelocytes + neutrophils. The percentage of immature granulocytes by flow cytometry showed good correlation with the manual percentage of blasts + promyelocytes + myelocytes. These results demonstrate that analysis of bone marrow aspirates using an automated hematology analyzer is valuable in the determination of bone marrow cellularity. Moreover, as flow cytometry provides objective findings for percentages of major cell populations, it can serve as a method to automate bone marrow differentials.