The etiology of acute lower respiratory tract infections (ARI) and nasopharyngeal bacterial carriage in children less than 5 years old living in Santa Fe city, Argentina, was studied. A total of 518 children were included in the study: 450 suffering from ARI and 68 asymptomatic children. Blood samples, pleural effusions and nasopharyngeal secretions (NS) were obtained from children for bacterial isolations. NS was also used for fluorescent antibody techniques, and serum samples were employed for detecting IgM anti Chlamydia trachomatis. A bacterial pathogen was isolated from blood in 6.2% (14/224) of the children with ARI. A total of 11 Streptococcus pneumoniae (five of them oxacillin resistant), two Haemophilus influenzae and one Staphylococcus aureus strains were isolated. The most frequently detected pathogen in the ARI group was respiratory syncytial virus (RSV). It was found in 23.3% (105/450) of the children with ARI. Among children with risk of Chlamydia trachomatis infection, 24% presented high titters of specific IgM antibodies. Main bacteria carried in NS in the ARI group were H.influenzae (31.6%) and S. pneumoniae (23.4%) while viridans streptococci (26.5%), H.influenzae (23.5%) and Moraxella catarrhalis (22.1%) were more frequently isolated from controls. The most common pneumococcal types were 14 and 7 and the main type of H.influenzae was b biotype I. During the period of this study, the susceptibility of the pneumococcal isolates to oxacillin decreased from 60% to 50.8%, and the H.influenzae susceptibility to ampicillin fell from 92.3% to 79%. All the H.influenzae type b isolations were susceptible to ampicillin.