Incidence of respiratory tract infection represents 23% of the total number of admissions between 1-24 months of age, during a period of 18 months. The diagnosis were: bronchiolities, 143 cases; bronchopneumonia, 134 cases; tracheobronchitis, 50 cases; laryngitis, four cases, and bacterial pneumonia, 61 cases. Monthly incidence was maximal in December of each year. From the total group, 144 cases were included in the present study to determine etiology of the infection. In 19% of the cases a serological diagnosis was posible. The adenovirus group was the most frequently found, followed by mycoplasma pneumoniae, parainfluenza 2, RS virus and M. parotiditis. RS virus was associated with a clinical picture of bronchopneumonia, mycoplasma pneumoniae with one of bronchiolitis and adenovirus was indistinctly associated with features either bronchopneumonia or bronchiolitis. In two cases it was detected a mixed infection by two virus: influenza 2 and mycoplasma pneumoniae. In four cases a bacterial surinfection was demonstrated: in two cases with coagulase-positive staphilococus and other two with klebsiella pneumoniae.