This study viral on participation in low acute respiratory tract infections (ARI) in 344 infants under 2 year of age was carried between years 1983 through 1985, by means of antigen detection and serological tests. A global positive isolation, was confirmed in 59.6% of cases (n = 205) with respiratory syncytial virus (RSV: n = 134; 65.4%), adenovirus (Ad: n = 41; 20.0%) and parainfluenza viruses (PI: n = 25; 12.7%) as the most commonly found agents. Mixed viral infections were detected in 16.6% of cases (n = 34). RSV showed the highest prevalence in all groups, mainly in patients under six months of age. PI increased with patient's age and similar but less noticeable raise ocurred in Ad infections. RSV was the only pathogen detected in infants with bronchial obstructive disease and was the prevalent organism in patients with all the other forms of disease. Ad and PI were equally found in all clinical syndrome without striking differences among each other. Influenza and citomegalovirus were also detected, but not clear evidence of pathogenic role was obtained for the later. Immunofluorescence was the best technique for RSV identification while isolation was better for Ad, The relevance of these viruses as causative agents of low ARI is confirmed and the need to further investigate other non viral etiologic agents is stressed.