BACKGROUND: Compared with traditional manual method, automated counting of reticulocytes offers greater sensitivity, improved accuracy, and less subjectivity. Also, automated blood cell analyzer enabled to measure immature reticulocyte fraction (IRF). In this study, we investigate the clinical significance of IRF in the evaluation of anemia. METHODS: Reticulocyte counts were measured with automated blood cell analyzer, Technicon H*3 (Bayer Diagnostics, USA) and manual method from 100 cases of anemic group and 20 cases of control group. Then, we compared the values of reticulocyte count between manual method and H*3. For investigate clinical significance of IRF in the evaluation of anemia, IRF determined by H*3 was compared with the absolute reticulocyte count (ARC) and with the reticulocyte production index (RPI). In addition, we subclassified anemic group into 5 different subgroups, such as hemolytic anemia or anemia due to hemorrhage, hematologic malignancy, anemia due to chronic renal failure, iron deficiency anemia, and anemia of chronic disorders. The value of ARC and IRF in each group was analysed. RESULTS: 1) The mean reticulocyte count by H*3 was higher than that by the manual method and the result of these two methods was correlated well (r=0.98, P<0.01). 2) IRF has a weak but significantly positive correlation (P<0.05) with ARC. IRF has insignificant correlation (P>0.05) with RPI. 3) Both ARC and IRF were increased in patients with hemolytic anemia or anemia due to hemorrhage. High mean value of IRF with reduced ARC were shown in hematologic malignancy. IRF were within normal range with reduced ARC in the group of chronic renal failure. Both IRF and ARC were within normal range in the group of iron deficiency anemia. In anemic group due to chronic disorder, ARC were within normal range with increased IRF. CONCLUSION: reticulocyte counting by the automated blood cell analyzer H*3 allows the improvement of accuracy and sensitivity. Immature reticulocyte fraction can be used for differentiation of various anemic disorders together with absolute reticulocyte count.