Examination of stained peripheral blood films for evaluation of erythrocyte and platelet morphology and leukocyte differential count provides considerable data for diagnosis of various diseases including hematologic disorders. Since 1980, the use of an automated leukocyte differentiation analyzer has become common in large clinical laboratories. We surveyed the principles, performance, and characteristics of the newest automated leukocyte differential instruments, GEN.S system (Coulter), SE-9000/RAM-1 (Toa), CELL-DYN 4000 (Dainabot), ADVIA120 (Bayer-Sankyo), VEGA RETIC LC-141 (Horiba), and MICROX HEG-120N (Omron). We also compared the differentiated function of GEN.S system and CELL-DYN 4000. Automated leukocyte differentiation using two instruments correlated well with a reference method for leukocyte differential counting proposed by the National Committee for Clinical Laboratory Standards (NCCLS) H20-T (eye-count differential). The sensitivities of the suspect flag in GEN.S system and CELL-DYN 4000 were 79.3% and 83.5%, and the specificities of the two instruments were 100% and 94.9%, respectively.