The aim of this study was to investigate the influence of the time, when measuring tidal breathing parameters 1 min (epoch 1) and 5 min (epoch 2) after application of the facemask in healthy infants and infants with bronchopulmonary dysplasia (BPD), using the dead space free flow-through technique. In both patient groups, there were no statistically significant differences between epoch 1 and 2, in most of the tidal breathing parameters, except an increased VE and increased correlation dimension of the respiratory signal in the BPD infants in epoch 1. However, in nearly all parameters the coefficient of variation (CV) was significantly higher in epoch 1 compared with epoch 2, and in some infants, we found very high CVs (>50%) in epoch 1, which disappeared in epoch 2. The study shows that after having applied the facemask, a sufficient amount of adaptation time is necessary in order to reduce the within-subject variability and improve the reproducibility and interpretation of tidal breathing measurements in infants.