Our main aim was to determine an appropriate time for lung function measurements in infants, prior to environmental influences upon their respiratory tract. Tidal flow-volume loops, respiratory system compliance (Crs) and resistance (Rrs) (single breath passive occlusion technique) were measured in 24 healthy, awake awake infants, at one hour and on the following four days of life to investigate variability and reproducibility over time. Possible differences in lung function were sought between the 12 vaginally-delivered and 12 Caesarean section-delivered infants. Tidal volumes increased each day, but significantly so only from Day 0 to Day 1. The expiratory flow-time ratios (time to reach peak expiratory flow to total expiratory time (Tme/Te) and tidal expiratory flow at 75% to peak flow (TEF25/PTEF)) and expiratory flow-volume ratio (volume to reach peak expiratory flow to total expiratory volume (Vme/Ve)) were significantly smaller on Day 1 than Day 0, but did not change significantly thereafter. Crs and Rrs were lower on Day 0 than later. Intra-individual variation remained stable for tidal flow-volume parameters throughout the study, but was significantly higher during Day 0 and Day 1 for Crs and Rrs. There were no significant differences related to mode of delivery of the infant. We conclude that, for epidemiological purposes, tidal lung function parameters may be measured from Day 2 to Day 4, and that they are not influenced by mode of delivery of the infant.