Acute respiratory tract infections (ARI) were studied during a 2-year period in 521 preschool children living in a marginal area of Guatemala City. There were 3,646 episodes of ARI detected during 26,329 child-weeks at risk, for an incidence of 14 per 100 childweeks or 7.2 episodes per child per year. The median duration of ARI episodes was 11 days. The highest incidence of ARI was observed in children 6–23 months old. Boys had more respiratory tract illnesses than did girls; the presence of a cigarette smoker in the household was associated with higher morbidity. Acute lower respiratory tract illnesses (ALRI) were more common among younger infants 0–5 months old, with nutritional status having no apparent effect. Parental formal education and crowding in households were found to be directly related to the incidence of ALRI. In general, ARI morbidity interfered with appropriate physical growth.