OBJECTIVE: To determine whether hospital length-of-stay (LOS) for bronchiolitis is influenced by the causative virus: respiratory syncytial virus (RSV) or rhinovirus. METHODS: This prospective study was carried out in 3 university hospitals in Finland during 2 consecutive winter seasons. We enrolled consecutive children <2 years of age hospitalized with an attending physician\'s diagnosis of bronchiolitis. All enrolled children were included in the primary analysis. A parallel analysis was also conducted using a stricter definition for bronchiolitis (age <12 months and no history of wheeze). Polymerase chain reaction was used to test the nasopharyngeal aspirate samples for multiple respiratory pathogens. RESULTS: The median age of the 408 children was 8 months, 73% had no history of wheeze and their median hospital LOS was 2 days. 144 (35%) children had RSV only and 92 (23%) children rhinovirus only infections. Children with rhinovirus only had shorter duration of prehospital symptoms, higher disease severity score at entry and more often a history of wheezing (all P </= 0.001). Controlling for 7 demographic and clinical characteristics, those with rhinovirus only had shorter hospital LOS when compared with children with RSV only (adjusted odds ratio: 0.45; 95% confidence interval: 0.22-0.92; P = 0.03). The rhinovirus only finding was similar in the subset of 206 children with stricter diagnosis (adjusted odds ratio: 0.30; 0.06-1.49; P = 0.14). CONCLUSIONS: Hospital LOS is associated with rhinovirus etiology of bronchiolitis. Our data call attention to the importance of both RSV and rhinovirus testing in clinical research.