Introduction: One of the most widespread viruses that infect humans and are spread by mosquitoes is dengue fever (DF), which is characterized by leucopenia and an increasing haematocrit of 5–10%. Even before thrombocytopenia, which develops after five days of fever, there are hematological alterations associated with dengue fever. Aims and objectives: To compare the total leucocyte count and monocyte count taken during the initial 3 days of fever and predicting the severity of dengue as well as predict the duration of hospital stay. Methodology: This ambidirectional (prospective and retrospective) longitudinal study was conducted over a period of 3 months in Department of General Medicine, Chettinad hospital and research institute on a total of 60 patients. Results: Leukopenia, normal WBC count and leukocytosis was observed in 60%, 33.33% and 6% of the study population respectively. Patients who had a more severe form of dengue showed higher rates of monocytosis. The length of hospital stay and the number of monocytes had a moderately positive and significant connection. Between the length of hospital stay and the total platelet count, a substantial negative connection was found. Conclusion: There was no statistically significant difference in leukocyte count or status between the severity and outcome of dengue. By using ROC analysis, monocyte count and percentage within the first three days after fever onset was found to be a more accurate indicator of dengue severity than total leukocyte count.