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 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.