ObjectiveTo know the effect of caffeine therapy on infant lung function in preterm infants with a gestational age less than 31 weeks.Material and MethodsForced vital capacity (FVC), forced expiratory volume at 0.5 seconds (FEV0.5), and forced expiratory flows were measured by raised volume rapid thoracoabdominal compression technique; functional residual capacity was measured by plethysmography (FRCpleth). Compliance of the respiratory system was measured by a single interruption technique (Crs). The Student t test was used to compare lung function measurements between the two groups: treated versus nontreated with caffeine. A multivariate analysis was carried out considering each and every lung function parameter (z‐score) as the dependent variable; and gender, gestational age, birth weight (z‐score), corrected age, invasive mechanical ventilation (yes/no), and bronchopulmonary dysplasia (BPD) diagnosis (yes/no) as independent ones. Additionally, stratified analyses by BPD diagnosis were performed.ResultsThe multivariate analysis showed significant higher z‐scores of FVC and FEV0.5 in preterm infants treated with caffeine (P = .004 and P = .024, respectively). This result only being significant in the group of non‐BPD infants (P = .021 and P = .042), after stratifying by BPD diagnosis. Differences were not found in z‐scores of FEV0.5/FVC, FEF75, FEF25‐75, FRCpleth, nor Crs.ConclusionLung function (FVC and FEV0.5) is improved in infants born under 31 weeks of gestation when treated with caffeine. This improvement is driven by the group of infants who did not suffer from BPD. Overall, our results show that there is an early beneficial effect of caffeine treatment in infant lung function

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