BACKGROUND: CELL-DYN 4000 System (Abbott Laboratories, IL, USA) is one of the pioneering instruments that applied flow cytometric technique to hematologic analyzer. This study was carried out to evaluate proficiency for routine complete blood count (CBC) with white blood cell (WBC) differentials and reticulocyte counting of the instrument. METHODS: The evaluation protocol was based on the recommendation of the ICSH (International Committee for Standardization in Haematology). Precision, linearity, carryover, sample preservation effect, comparability for CBC with WBC differentials and reticulocyte counting were evaluated in the study. RESULTS: For precision, all parameters investigated showed the coefficient of variation (CV) values similar to those of manufacturer's. The storage effect was not significant until 10 hours after sampling but the significant change was observed after that at room temperature in WBC, RBC, MCV and RDW. To store samples before analysis for more than 10 hours, it should be stored at 4degrees C. The Linearity was excellent for WBC, red blood cell (RBC), and hemoglobin with R2 over 0.985. And that for platelet was good with R2 value of 0.96. Carryover was significantly lower than that of manufacturer's with less than 0.54% for WBC, RBC, hemoglobin and reticulocyte. It showed good correlation with ADVIA 120 (Bayer Corporation, Tarrytown, NY, USA) especially in parameters such as WBC, RBC, hemoglobin, platelet, and red cell distribution width (RDW). Very good precision, carryover and linearity were found in reticulocyte counting. CONCLUSIONS: CELL-DYN 4000 System showed excellent precision, linearity, and comparability and low carryover in the parameters evaluated. And we found out it was a good instrument for routine hematology laboratory work including CBC with WBC differentials and reticulocyte counting.