The diagnosis of myelodysplastic syndromes (MDS) without an increase in blasts and ringed sideroblasts (low-grade MDS without ringed sideroblasts [LGw/oRS]) may be problematic because dysplastic features are not specific to MDS and approximately 50% of patients with LGw/oRS lack chromosomal aberrations. Here, we report the usefulness of flow cytometric characteristics of CD34(+) cells for LGw/oRS diagnosis. Bone marrow cells from LGw/oRS patients and controls (eg, cytopenic individuals without MDS) were analyzed using 4-color flow cytometry (FCM). We objectively determined reference ranges of 13 parameters related to CD34(+) cells with data from controls. In LGw/oRS patients, various abnormalities of CD34(+) cells-eg, decrease in CD34(+) B-cell precursors, aberrant expression or overexpression of various antigens on CD34(+) myeloblasts-were observed. We constructed a reproducible, flow cytometric scoring system for LGw/oRS diagnosis. High scores were observed in 16 of 27 LGw/oRS patients, regardless of the presence or absence of chromosomal aberrations, but not in any of the 90 controls. Among LGw/oRS patients with chromosomal aberrations, patients with trisomy 8 or del20(q) had low FCM scores (P = .002). As a result, most LGw/oRS patients were identified based on high FCM score, chromosomal aberration, or both.