OBJECTIVE: To identify the viruses involved in acute respiratory tract infections and to analyze the rates of hospitalization and death in children on palivizumab prophylaxis. METHODS: Prospective cohort of 198 infants up to one year old who were born before 29 weeks of gestational age and infants under two years old with hemodynamically unstable cardiopathy or chronic pulmonary disease who received prophylactic palivizumab against severe respiratory syncytial virus infections in 2008. During the study period, in each episode of acute respiratory tract infection, nasopharyngeal aspirate was collected to identify respiratory syncytial virus, adenovirus, parainfluenza 1, 2 and 3, influenza A and B by direct immunofluorescence, rhinovirus and metapneumovirus by polymerase chain reaction preceded by reverse transcription. Data regarding hospitalization and deaths were monitored. RESULTS: Among the 198 studied infants, 117 (59.1%) presented acute respiratory tract infections, with a total of 175 episodes. Of the 76 nasopharyngeal aspirates collected during respiratory tract infections, 37 were positive, as follow: rhinovirus (75.7%), respiratory syncytial virus (18.9%), parainfluenza (8.1%), adenovirus 2 (2.7%), metapneumovirus (2.7%) and three samples presented multiple agents. Of the 198 children, 48 (24.4%) were hospitalized: 30 (15.2%) for non-infectious etiology and 18 (9.1%) for respiratory causes. Among these 18 children, one case of respiratory syncytial virus was identified. Two deaths were reported, but respiratory syncytial virus was not identified. CONCLUSIONS: During the prophylaxis period, low frequency of respiratory syncytial virus infections and low rates of hospitalization were observed, suggesting the benefit of palivizumab prophylaxis.