In January 1980, the municipal area of Rayong, Thailand, and contiguous suburban villages were chosen for a long-term study on dengue epidemiology. From 3,185 children randomly sampled in schools and households, the population prevalence of neutralizing antibody to the four dengue serotypes was estimated. To estimate the incidence of infection with each dengue virus serotype (dengue seroconversions), first grade children were re-bled in January 1981 (cohort study). Children admitted to hospital were studied for dengue virus isolation and antibody responses in paired sera. An epidemic of dengue occurred in 1980. Plaque reduction neutralization tests of 1,009 pre-epidemic sera from children aged less than 1-10 years of age determined that 3.3% were immune to dengue 1, 13.2% to dengue 2, 6.4% to dengue 3, and 5.8% to dengue 4. Examination of pre- and post-epidemic cohort blood samples revealed that the incidence of dengue infection in 251 seronegative children was 39.4% (15.1% dengue 1, 11.1% dengue 2, 2.0% dengue 3, 4.8% dengue 4, and 6.4% two or more dengue viruses). Among the 52,935 residents of the study area, there were 22 cases of virologically and clinically confirmed dengue shock syndrome, in children 15 years or younger. All 22 shock syndrome cases had secondary type antibody responses. Eight of 22 had been included in the random serologic sample prior to onset of shock; five had been immune to dengue 1, two to dengue 3, one to dengue 4, and none to dengue 2. Despite the high rate of dengue 1 infections in 1980, only dengue 2 viruses were recovered from dengue shock syndrome cases, including two dengue 1 immune children with pre-illness serum specimens. Although the pre-epidemic prevalence of antibodies to dengue 1 was the lowest to any type, children with this immunologic background contributed disproportionately to shock cases. In descending order of magnitude, risk factors for dengue shock syndrome in Rayong were secondary infections with dengue 2 which followed primary infections with dengue 1, dengue 3, or dengue 4.