The aim of this study was to determine the distribution of blood group type in diagnosed NS1Ag positive dengue cases. TheABO phenotypes were identified by use of blood from patients with dengue virus infection prospectively, in newly diagnosed patients at a tertiary care hospital in south India. A total of 162 NS1Ag positive cases were collected with dengue illness. The greatest number (25%) of dengue cases were seen in the11-20 year age group. Males were predominantly effected(58%).It was found that majority(45%) of cases belonged to blood groupO.