Bronchiolitis is the most common cause of hospitalization of children in the first year of life. The lung ultrasound is a new diagnostic tool which is inexpensive, non-invasive, rapid, and easily repeatable. Our prospective study was conducted in the emergency department and all patients underwent a routine clinical evaluation and lung ultrasound by the pediatricians who defined the clinical and the ultrasound score. We enrolled 76 infants (median age 90 days [IQR 62-183], 53.9% males). In nasopharyngeal aspirates, the respiratory syncytial virus was isolated in 33 patients. Considering the clinical score, children with higher score had a higher probability of requiring respiratory support (p 0.001). At the ultrasound evaluation, there was a significant difference on ultrasound score between those who will need respiratory support or not (p 0.003). Infants who needed ventilation with helmet continuous positive airway pressure had a more severe ultrasound score (p 0.028) and clinical score (p 0.004), if compared with those who did not need it.Conclusion: Our study shows that lung ultrasound in the bronchiolitis may be a useful method to be integrated with the clinical evaluation to better define the prognosis of the individual patient. Multicenter studies on larger populations are necessary to confirm our data. What is Known: • Bronchiolitis is the main cause of lower respiratory tract infection in children younger than 24 months. • Ultrasound can evaluate the lung parenchyma without ionizing radiations. What is New: • Lung ultrasound may be a useful diagnostic tool to define the prognosis of the infants affected by bronchiolitis if performed at the first assessment in the emergency department. • The score obtained at the ultrasound evaluation is higher in those who will need oxygen therapy during admission for more time and in those who will need respiratory support with helmet continuous positive airway pressure.