Comparative analyses of the leukocyte differential counting were performed using a Coulter VCS Hematology Flow Cytometer and direct microscopic observation on 547 unselected individuals analyzed at the outpatient clinic of the Institute of Hematology "L. e A. Seràgnoli" of Bologna. The Coulter VCS is able to provide leukocyte differential counting by measuring cell volume, conductivity and laser light scatter. Negative (true negative: 50.8%) results were observed in 278 subjects by both automated and direct observation methods. Abnormal cells in the blood samples (true positive results) were detected both by VCS and by direct observation in 199 cases (36.4%). Regional distribution of the cells in specific "flags" on the scatterplot was often associated with specific cell types. In 66 cases (12.1%) no abnormal cell was detected by direct observation, while Coulter VCS gave an abnormal pattern, even if only to a slight extent (false positive cases). In 4 cases only (0.7%), false negative results were given by the VCS system. The correlation of the results given by the VCS system with those given by direct microscopic analysis was very high; however careful control by the operator was essential in evaluating the data given by the automated system and in identifying the type of abnormal cells detected.