The pathogenesis of hematologic changes in dengue patients is not clearly understood. Consistent hematological findings include vasculopathy, thrombocytopenia, and coagulopathy. There are evidences suggesting that dengue virus causes pathophysiological changes that involve all of the consistent hematologic findings resulting in vasculopathy, reduction in platelet number as well as platelet dysfunction, and reduction of several coagulation factors. Laboratory evidences of disseminated intravascular coagulation (DIC) are also demonstrated in all degrees of severity in dengue patients. Only in severe dengue cases is profound DIC aggravated, leading to uncontrolled bleeding and death. A study to determine the extent of the activation of endothelial cells and the hemostatic system in correlation with clinical severity and also to detect the best prognostic factor for severe dengue showed plasma von Willebrand factor antigen (VWF:Ag) to be the best indicator of progression to severe dengue.