Serum specimens collected during a prospective study of dengue infections among schoolchildren in Bangkok were tested for their ability to enhance dengue 2DEN-2 virus growth in human monocytes in vitro. Two groups of dengue-immune sera were compared 32 dengue antibody positive serum specimens from children who subsequently developed asymptomatic secondary dengue infections and 9 dengue antibody positive serum specimens from children who subsequently developed severe symptomatic secondary dengue infections, 8 of which were clinically diagnosed as dengue hemorrhagic fever. Dengue antibodies can be neutralizing and therefore protective, or they can be enhancing and increase the risk of dengue hemorrhagic fever. In the current study, we sought to determine if differences in serum ADE activity in preinfection sera from seropositive children could be related to the severity of subsequent infection. We report here that, among dengue-immune children, high serum DEN-2 ADE activity was a strong predictor of severe illness. Reprints.