There is a need for a rapid and reliable test to predict dengue so that the patients are managed and monitored. Beckman Coulter LH series instruments provide data on leukocyte cell volume, V; conductivity, C; and light scatter, S which is useful in distinguishing viral and bacterial infections. This prospective study was undertaken in Chennai - India for a period of one and a half years. The cell population data for the dengue positive and negative patients were retrieved from the hematology analyzer Beckman Coulter LH780. Statistical analysis was performed using MedCalc for Windows version 15.0. Mann-Whitney test was used to compare the VCS indices between the different groups. Dengue positive, suspected and controls were 499, 493, 499 respectively. Male: female ratio is 1.4:1. The platelet count, standard deviation of volume and conductivity of monocytes, volume and percentage of lymphocytes and platelet counts were used to construct a tree model which distinguishes dengue from suspected cases with sensitivity, specificity and efficiency of 94.84%, 77.88% and 89.59% respectively. A classification tree was developed using changes in monocytes, lymphocytes and lymphocyte and platelet counts.