Purpose: We investigated the influence of respiratory virus and atopic characteristics on the severity of bronchiolitis. Methods: Four hundred and eighteen infants <2-years-old and hospitalized at Daegu Fatima Hospital with bronchiolitis from March 2007 to February 2010 were evaluated. They were detected for specific respiratory viruses in nasopharyngeal aspirates by multiplex reverse transcription-polymerase chain reaction. Clinical severity score, based on respiratory rate, wheezing, chest retraction, and oxygen saturation was assessed at admission. According to the scores, all patients were divided into a mild to moderate bronchiolitis group and a severe group. Clinical data related to host factors, including atopic characteristics and respiratory viruses, were compared among individual groups. Multivariate logistic regression analyses were performed to identify independent risk factors for severe bronchiolitis. Results: A single virus was identified in 365 infants (87%) and multiple viruses in 53 (13%). Respiratory syncytial virus (RSV) was the most common virus detected (51%). RSV and rhinovirus were the viruses most frequently identified in mixed infections in infants hospitalized with bronchiolitis. Infants with coinfections were 3.28 times (95% confidence interval, 1.36 to 7.89) more risk for severe bronchiolitis than those with a single infection. Host factors associated with more severe bronchiolitis included male gender, younger age, prematurity, and chronic cardiorespiratory illness. Type of viruses, personal and family history of atopy, and passive smoking were not significantly associated with bronchiolitis severity. Conclusion: Viral coinfections and host factors, including male gender, younger age, prematurity, and chronic cardiorespiratory illness are relevant risk factors for infants with severe bronchiolitis. (Pediatr Allergy Respir Dis(Korea) 2011;21:302-312)