BACKGROUND The role of respiratory viruses in the pathogenesis of bronchiolitis was re-evaluated with the use of molecular methods such as PCR for virus detection. Whether specific viruses or the classical clinical risk factors are more important in determining severe bronchiolitis is not well established. AIM To analyze the specific viruses and clinical variables that can predict severe bronchiolitis at admission. METHODS Nasopharyngeal aspirates were prospectively collected from 484 children <12 months admitted to the pediatrics ward or PICU at Universitary Hospital Sant Joan de Déu (Barcelona, Spain) for bronchiolitis from October 2007 to October 2008. Clinical and demographic data were collected. Sixteen respiratory viruses were studied using PCR. Severity was assessed with a bronchiolitis clinical score (BCS). RESULTS Four hundred ten infants that tested positive for respiratory viruses were analyzed. Mixed viral infections did not increase the severity of the disease. Rhinovirus was associated with severe BCS in univariate analysis (P = 0.041), but in the multivariate logistic regression including viruses and clinical data only bronchopulmonary dysplasia (OR 7.2; 95% CI 1.2-43.3), congenital heart disease (OR 4.7; 95% CI 1.1-19.9), prematurity (OR 2.6; 95% CI 1.3-5.1), and fever (OR 1.8, 95% CI 1.1-3.1) showed statistical significance for predicting severe BCS. CONCLUSIONS Classical clinical risk factors have more weight in predicting a severe BCS in infants with acute bronchiolitis than the involved viruses.