BACKGROUND: Acute bronchiolitis frequently causes infant hospitalization. Studies on different viruses or viral genomic load and disease severity or treatment effect are conflicting. We aimed to investigate if airway viruses were associated with disease severity or the treatment effect of inhaled adrenaline. METHODS: Nasopharyngeal aspirates were collected in 363 infants with acute bronchiolitis in an RCT that compared inhaled racemic adrenaline versus saline. Virus genome was identified and quantified by PCR analyses. Severity was assessed by the length of stay(LOS) and the use of supportive care. RESULTS: Respiratory syncytial virus (RSV) (83%) and Human rhinovirus (34%) was most commonly detected. Seven other viruses were present in 8-15% of the patients. Two viruses or more (maximum seven) were detected in 61% of the infants. Virus type or confection was not associated with disease severity. A high genomic load of RSV was associated with a longer LOS and increased use of oxygen and ventilatory support. Treatment effect of inhaled adrenaline was not modified by virus type, load or coinfection. DISCUSSION: In infants hospitalized with acute bronchiolitis, disease severity was not associated with specific viruses or the total number of viruses detected. High genomic load of RSV was associated with more severe disease.