Two major epidemic waves of dengue 1 occurred in Rio de Janeiro, Brazil, in 1986 and 1987. This article reports the results of a survey of Rio schoolchildren who were tested serologically for antibodies to dengue 1 before and after the second major epidemic wave. The highest percentages of positive subjects were found in districts with relatively poor socioeconomic conditions and mosquito control problems. It also appears likely that the estimated number of dengue cases occurring in 1986-1987 was substantially below the number that actually occurred. In addition, because of this exposure to dengue 1, Rio de Janeiro now runs the risk of dengue hemorrhagic fever/dengue shock syndrome occurring should another dengue serotype be introduced within the next few years.