A 2-year longitudinal study was conducted among the population of a socioeconomically depressed urban community in Bangkok, Thailand, from January 1986 through December 1987 to determine the incidence, etiologic agents, and risk factors associated with acute respiratory tract infection (ARI) in children less than 5 years of age. Data were obtained for a total of 674 children, who were visited twice weekly for detection of signs and symptoms of ARI. During the first year of the study, throat-swab specimens were obtained for bacterial culture from both ill and healthy children and a nasal wash was performed on mildly ill children for detection of virus. During both years of the study, nasopharyngeal aspiration for identification of virus was performed for children with more severe infection. The overall incidence of ARI was 11.2 episodes per child-year. The highest (14.9) and lowest (8.8) rates per child-year occurred in age groups 6-11 months and 48-59 months, respectively. Respiratory syncytial virus, parainfluenza virus, adenovirus, Streptococcus pneumoniae, and Haemophilus influenzae were the prevalent pathogenic agents identified. Factors associated with higher risk of ARI were low family income, working mothers, mothers with allergies, chronic malnutrition, and crowding in the home.