The increasing availability of appliances for measuring lung function in infants may allow clinical and epidemiological applications. The aim of the present study was to establish reference values for tidal breathing lung function in awake newborn infants and to investigate potential sources of variability. Tidal flow-volume loops were measured in 803 awake, healthy infants (427 males and 376 females) and passive respiratory mechanics (single-breath occlusion technique) in 664. Mean postnatal age was 2.7 +/- 0.9 (sd) days, gestational age 39.8 +/- 1.4 weeks and birthweight 3.59 +/- 0.49 kg. Tidal expiratory volume (Vt), peak tidal expiratory flow (PEF), and mid-expiratory flow increased significantly with increasing birthweight. Flow ratios: ratio of time to PEF to total expiratory time (Tpef/Te), ratio of volume to PEF to total expiratory volume (Vpef/Ve); and ratio of tidal flow at 25% remaining expiration to PEF (TEF25/PEF), were highest in 1 day old infants (medians 0.39, 0.46 and 0.81 respectively), decreasing to a minimum in 4-5 day old infants, but were not influenced by birthweight. Tidal flows and flow ratios were higher in males versus females, even after weight adjustment. Respiratory rates correlated significantly with tidal flows (r = 0.66), inversely with Vt (r = 0.40), but not with flow ratios. Mean compliance of the respiratory system was 1.18 ml.cmH2O-1.kg birthweight (95% confidence interval (95% CI) 1.15-1.21) and mean resistance 0.051 cmH2O.ml-1.s (95% CI 0.049-0.054). These results demonstrate that lung function in awake healthy infants varies according to weight, gender and postnatal age.(ABSTRACT TRUNCATED AT 250 WORDS)