In a dengue-endemic area, sera from 42 mother-infant pairs and sera collected from the infants at follow-up at the ages of 3 months (n =27), 6 months (n =34), 9 months (n =23) and 12 months (n =8) were tested for antibodies to four dengue serotypes using a plaque reduction neutralisation test (PRNT(50)), IgG ELISA and haemagglutination inhibition assay (HAI). The IgG ELISA and HAI tests were less sensitive than PRNT(50) in detecting low levels of antibodies. Levels of maternally transferred dengue neutralising antibodies in the cord sera were very high and identical to those in the mothers. Neutralising antibody prevalences in the newborns were 95% to dengue serotype 1 (DEN-1), 93% to DEN-2 and DEN-3 and 91% to DEN-4. The antibodies decreased with increasing age. At least one dengue serotype neutralising antibody persisted in the infants at ages 3, 6 and 9 months in 92%, 69% and 13%, respectively. No maternally transferred antibody was observed in 12-month-old infants. In endemic areas therefore, where most infants have maternally transferred dengue antibodies, interference with dengue vaccine is likely to be less after 12 months of age than before then.