Introduction: lower acute respiratory infection (LRTI) is one of the most important causes of morbidity and mortality in children <5 years especially in low- or middle- income countries. Epidemiological studies of respiratory syncytial virus (RSV) and adenovirus are scarce in these countries. Methods: in a retrospective observational cohort study, we reviewed the medical records of all patients hospitalized for LRTI, RSVpositive and / or adenovirus, between May 1, 2009 and May 31, 2011. Predictors were evaluated for severe disease and mortality. Results: Of 3.137 children hospitalized for LRTI with RSV and adenovirus samples, were included 1.338 (42.7%) positive for at least one of the two viruses. Age <6 months (IRR: 1.92, 95% CI: 1.59-2.32, p <0.001), history of previous lung disease (IRR: 1.84, 95% CI: 1.33-2.54, p <0.001), history of prematurity (IRR: 1.48, 95% CI: 1.22-1.81, p <0.001), congenital heart disease (IRR: 1.84, 95% CI: 1.33-2.54, p =0.002), having taken the sample of nasopharyngeal aspirate in a different month 3-month period between March and May (IRR: 1.21, 95% CI: 1.00-1.47, p = 0.043), and mixed infection with RSV and adenovirus (IRR: 1.79, 95% CI: 1.34-2.39, p <0.001) were independent predictors of severe disease. Cancer as a comorbidity was the only independent predictor of mortality (IRR: 25.87, 95% CI: 6.05-110.59, p <0.001). Conclusions: RSV and adenovirus are an important cause of LRTI in children under 36 months in the city of Bogota, especially in March, April and May. We identified independent predictors of severe disease and death.