Tidal breathing flow-volume loops were recorded in 19 healthy newborn infants when awake and asleep. This preceded and followed measurements of passive lung mechanics (by single breath occlusion). Our aim was to evaluate possible differences in lung function due to state of arousal or any influence of the occlusion technique. Expiratory volumes and flow rates were larger in awake than in sleeping infants before, but not after occlusion measurements. In sleeping, but not in awake infants, expiratory volumes and flow rates were higher after occlusion than before. Respiratory system compliance was significantly larger in sleeping than awake infants, while differences in respiratory system resistance and airway plateau pressure did not reach a significant level. Our results show that lung function can be measured in awake as well as sleeping infants, but differs significantly according to their arousal state, and whether tidal expiratory flow measurements are performed before or after airway occlusion measurements. Separate reference values for awake and sleeping infants may, therefore, be required. Marked intrasubject variability was found in the occlusion measurements, and criteria for acceptable measurements need to be defined.