A community-based study of acute respiratory tract infections in children in Uruguay. Acute respiratory tract infection (ARI) was investigated in children less than 5 years old in a longitudinal community-based study of 166 families living in a socioeconomically depressed area in Montevideo, Uruguay. Pediatricians made home visits every 10 days from May 1985 to December 1987, and symptoms and signs of ARI were recorded. The incidence of ARI was 5.8 episodes per child-year during the first 12 months of life and decreased with increasing age of the index children; the rate was highest in children 1-5 months old. Children observed from birth were ill during 21% of the visits. According to the definitions of the study, the incidence of lower respiratory tract infection was 11.6% higher than the incidence of upper respiratory tract infections. The rates of ARI were higher during the colder months. Most risk factors for ARI were only marginally statistically significant. 