Background: We investigated whether combining lung ultrasound scores (LUSs) and respiratory system reactance (Xrs) measured by respiratory oscillometry explains the severity of lung disease better than individual parameters alone.Methods: We performed a prospective observational study in very preterm infants. Forced oscillations (10 Hz) were applied using a neonatal mechanical ventilator (Fabian HFOi, Vyaire). We used the simultaneous respiratory severity score (RSS = mean airway pressure x FIO2) as a primary outcome. We built linear mixed-effect models to assess the relationship between Xrs z-score, LUS and RSS and compared nested models using the likelihood ratio test (LRT).Results: We enrolled 61 infants (median (Q1, Q3) gestational age ...