Background: Acute viral bronchiolitis (AVB) is a very frequent disease that affects the lower airways of young children increasing the inspiratory and expiratory resistance in variable degree as well as reducing the pulmonary compliance. It would be desirable to know whether these variables are associated with the outcome. Objectives: To evaluate the respiratory mechanics in infants with AVB requiring mechanical ventilation (MV) support and to evaluate if respiratory mechanics predict outcomes in children with AVB supported on MV. To evaluate the respiratory mechanics in infants with AVB submitted to MV. Materials and methods: A prospective observational study was conducted in two pediatric intensive care units (PICUs) between February 2016 and March 2017. Included were infants (1 month to 1 year old) admitted with AVB and requiring MV for >48 hours. Auto-PEEP, dynamic compliance (Cdyn), static compliance (Cstat), expiratory resistance (ExRes), and inspiratory resistance (InRes) were evaluated once daily on the second and third day of MV. Results: A total of 64 infants (median age of 2.8 months and a mean weight of 4.8 ± 1.7 kg) were evaluated. A mean positive inspiratory pressure (PIP) of 31.5 ± 5.2 cmH₂O, positive end-expiratory pressure (PEEP) of 5.5 ± 1.4 cmH₂O, resulting in a mean airway pressure (MAP) of 12.5 ± 2.2 cmH₂O and delta pressure of 22.5 ± 4.4 cmH₂O without difference between the two hospitals. Measurements of respiratory mechanics showed high values of InRes and ExRes (median 142 [IQ_25–75 106–180] cmH₂O/L/s and 158 [IQ_25–75 130–195.3] cmH₂O/L/s, respectively), accompanied by decreased Cdyn and Cstat (0.46 ± 0.19 and 0.81 ± 0.25 mL/kg/cmH₂O, respectively). None of the variables was associated with mortality, length of MV, or length of PICU stay. Conclusion: Infants with AVB requiring MV support present very high InRes and ExRes values. These findings might be the reason for the aggressive ventilatory parameters, especially PIP, required to ventilate this group of children with lower airway obstruction. Clinical significance: Monitoring respiratory mechanics could represent a useful tool to guide the ventilatory strategy to be adopted in patients with AVB.