BACKGROUND Acute lower respiratory infection (ALRI) is one of the main causes of morbidity and mortality in small children. OBJECTIVE The aim of this study was to determine the frequency, seasonality and association with clinical entities of respiratory syncytial virus (RSV) and adenoviruses in children with ALRI. METHODS During 2 consecutive years (1991-1992), 168 children under 2 years of age hospitalized due to ALRI in a public pediatric hospital of Buenos Aires, Argentina, were studied. RSV and adenoviruses were investigated on nasopharyngeal aspirates (NPA) by indirect immunofluorescence (IIF). HEp-2 cells were used for adenovirus isolation. RESULTS RSV was detected in 36.3% and adenoviruses in 14.3% of the cases (P < 0.0001). All adenoviruses detected by IIF were also isolated in culture. Out of 61 RSV cases, 57% corresponded to bronchiolitis and 43% to pneumonia. Ninety-two per cent of children with RSV were less than 1 year old and 70% were less than 5 months. The highest number of RSV cases were observed during winter, with a clear peak in July. Seventy-one per cent of adenovirus cases were associated with pneumonia and only 24% with bronchiolitis (P < 0.02), and predominated in children older than 5 months of age (P < 0.0001). Adenoviruses were detected in almost all months of the year with a small peak at the end of winter and beginning of spring. No significant differences in clinical features at admission, breast feeding or malnutrition were observed among children with RSV or adenovirus diagnosis versus those with no viral etiology. The overall fatality rate was 2.4%. In all fatal cases adenovirus was detected in NPA. Thus, fatality rate among patients with adenoviruses reached 16.7%. CONCLUSIONS Our findings show the importance of RSV and adenoviruses associated with ALRI in hospitalized children under 2 years of age and the different epidemiological patterns of the two viruses in Buenos Aires, Argentina.