The R-3000 reticulocyte analyzer uses flow cytometry with an argon laser as its light source. This analyzer stains residual RNA with auramine O to provide a reticulocyte maturation differential. Using the R-3000, we analyzed 119 samples of bone marrow (BM) and peripheral blood (PB) from 111 patients with hematologic disorders. Parameters were reticulocytes, immature reticulocyte fraction (IRF) percentage in BM and PB, BM/PB reticulocyte ratio, and BM/PB IRF ratio. Reticulocytes and IRF percentage in BM were significantly higher than in PB (p < 0.01). There was also a good correlation between reticulocyte percentages in BM and in PB (r = 0.81). Patients were classified into a normal group (without anemia) and an anemia group. Furthermore, the anemia group was classified into three groups: group 1: cases with hematopoietic dysfunction; group 2: cases in bone marrow recovery phase after chemotherapy and hematopoietic stem cell transplantation, and hematologic disorders with bone marrow accelerative phase, and group 3: cases with ineffective hematopoiesis (myelodysplastic syndrome). The mean reticulocyte percentage of the normal group was 2.3 ± 1.1%, which was close to the normal value in BM. The BM/PB reticulocyte ratio of group 3 was statistically higher than that of groups 1 and 2. This indicates that group 3 had ineffective erythropoiesis and that the BM/PB ratio is a useful indicator for the diagnosis of myelodysplastic syndrome.