This study was aimed to investigate the characteristics of immunophenotypes in the patients with myelodysplastic syndrome (MDS) without an increase of marrow blasts, and to confirm their diagnostic significance. Marrow cells from 222 patients with pancytopenia, dysplastic changes in one or more hematopoietic lineages and blast cells less than 5% were analyzed by multiparametric flow cytometry(FCM). The abnormal immunophenotypes were evaluated in asynchronous antigen expression (CD34 or CD117 in mature granulocytes or mature monocytes, HLA-DR in mature granulocytes), in cross-lineage antigen expression (CD7 or CD56 in granulocytes or monocytes), in aberrant light-scatter (CD45/SSC in mature granulocyte or monocyte) and in abnormal expression of differentiation antigen (CD13/CD16 pattern in granulocytes and HLA-DR under-expression in monocytes). The sensitivity and specificity of abnormal immunophenotypes were determined on diagnosis. Among 222 cases, 127 cases were diagnosed as MDS by traditional diagnostic method and 95 cases were non-MDS (drug-related neutropenia, autoimmune cytopenia and idiopathic thrombocytopenia). In mature granulocyte gate, the sensitivity of asynchronous, cross-lineage antigen expression, aberrant light-scatter of CD45/SSC and abnormal expression of differentiation antigen were 31.5%, 30.7%, 49.6% and 60.6% respectively, and the specificity were 100%, 100%, 88.4% and 52.6% respectively. In monocyte gate, the sensitivity of asynchronous, cross-lineage antigen expression, aberrant light-scatter of CD45/SSC and abnormal expression of differentiation antigen were 2.3%, 11%, 37% and 12.6% respectively. The specificity was 100% in all of them. Among 8 above mentioned items, sensitivity of more than 2 abnormalities was 77.9%, and specificity was 95.8%. The positive predictive value was 96.1%. It is concluded that the abnormal expression of asynchronous, cross-lineage antigen expression, aberrant light-scatter of CD45/SSC have a high specificity and a low sensitivity for diagnosis of MDS. The abnormal expressions of differentiation antigens have a high sensitivity and a low specificity; however, the detection of multiple expression abnormalities possesses the high sensitivity and specificity for diagnosis of MDS.