Background: A study was undertaken to examine factors that might influence lung function during infancy and to test the hypothesis that change in weight during infancy is negatively associated with change in lung function. Methods: Weight, length and maximal flow at functional residual capacity (V9maxFRC) were measured at ages 1and 12 months. V9maxFRC was adjusted for length. Asthma symptoms and age at introduction of formula feeds were identified from questionnaires. Groups were dichotomised by V9maxFRC at 1 month and change in V9maxFRC. Results: 154 infants were assessed at ages 1 and12 months. The change in V9maxFRC was inversely associated with change in weight (r=20.18, r2=0.13, p,0.001). The group with lower V9maxFRC at 1 month and reduced change in V9maxFRC over infancy had thegreatest weight gain (p=0.003) and increased risk for asthma symptoms by 3 years (p=0.017) but not afterwards. Exclusive breast feeding to 6 months was associated with a mean reduction in weight gain at age12 months in comparison with earlier introduction of formula milk (mean difference 0.65 kg, p=0.001), and was also associated with reduced asthma symptoms at 3 years (odds ratio 0.44, p=0.043) but not at 6 or 11 years of age. Conclusions: Weight gain in infancy is inversely associated with change in lung function during infancy.Postnatal weight gain may be indirectly associated with early transient asthma symptoms via an influence on lung growth during infancy, and this is potentially modifiable by breast feeding. These associations could be relevant to the clinically recognised syndrome of the ‘‘fat happy wheezer’’.