Objective: Dengue virus infection has diverse clinical manifestations often with unpredictableclinical evolution and outcome. This study was to evaluate the value of the peripheral blood cellin association with the severity of Dengue Infection. Method: This was an observational study on480 adult patients admitted to Bach Mai Hospital (Vietnam) with Dengue illness between Augustand October 2017. The diagnosis and classification of Dengue were based on WHO 2009 criteria.Blood samples were collected from patients during acute febrile phase time in hospital and theblood cell indices were analyzed. Results: A total 480 patients participated in the study including223 patients of Dengue without warning signs (A), 212 patients of Dengue with warning signs (B),and 45 patients of severe Dengue (C). B and C were considered as the severe group, and A wasclassified as the non-severe group. Compared to the non-severe group, the severe group hadsignificantly lowered hemoglobin, WBC, lymphocytes count and percentage of lymphocyte duringacute febrile phase on day 5 of illness. The percentagelymphocyte <25% was associated with the severe group. Conclusion: The percentage oflymphocytes, percentage of neutrophil are likely to predict for severity.