To assess the incidence of acute respiratory infections and bacterial colonization in children attending a daycare center.A cohort study was conducted from April to Octuber 1999, among 85 children aged under four years, who attended the daycare center at Hospital Infantil de Mexico (Mexico City's Children's Hospital) "Federico Gómez". Acute respiratory infection incidence rates and quarterly point prevalence figures of nasopharyngeal colonization were obtained. Data were analyzed using descriptive statistics.A total of 85 children were studied (40 girls and 45 boys) during 9,090 children-days of follow-up. Three children had a history of atopia (3.5%), six a history of asthma (7%), and 39 (46%) were exposed to passive smoking. There were 258 events of respiratory tract infection for an incidence rate of 10.3 infections per person-year (95% CI 8.7-12.0). The main clinical syndromes were pharyngitis (95%), acute otitis media (3.5%), and bronchiolitis (1%). The incidence rates of otitis and bronchiolitis were 0.36 and 0.12 per child-year of observation, respectively. The prevalence figures of nasopharyngeal colonization for the three main bacteria were: S. pneumoniae 20.4%; nontypable H. influenzae 13%; and Moraxella catarrhalis 8%.Study results show a high prevalence of colonization due to invasive strains, as well as a two-fold incidence rate of acute respiratory infection, higher than those reported in community surveys. These results add to the description of this poorly documented infectious disease in Mexico. The English version of this paper is available at: http://www.insp.mx/salud/index.html.