Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions. BACKGROUND: Our hypothesis was that factors associated with wheeze will be associated with changes in lung function trajectory between 1 month and 18 years of age. METHODS: Measurements of lung function were made in individuals aged 1, 6 and 12 months (V'maxFRC), and also at ages 6, 12 and 18 years (FEF(25-75)). Changes in lung function over time relative to sex, a history of asthma, maternal asthma and other factors were explored using random coefficient models. RESULTS: Lung function (maximal flow at functional residual capacity in infants and FEF(25-75) in children) was determined in 241 individuals at 1 month, 192 at 6 months, 164 at 12 months, 106 at 6 years, 183 at 12 years and 141 at 18 years. In the multivariable model, maternal asthma (mean reduction in lung function 9.8%), flow limitation (mean reduction 17.4%), infant atopy (mean reduction 12.6%) and maternal smoking (mean reduction in lung function 8.1%) were acting independently. When interactions with time were sought, the reduction in lung function associated with maternal asthma and infant atopy were consistent over time, but % lung function increased in boys by a mean of 1%/year compared with girls, in flow-limited individuals by 3.0%/year and by 0.9%/year for those exposed to maternal smoking during pregnancy compared to other cohort members. CONCLUSIONS: Decrements in lung function in 18-year-olds associated with maternal asthma and early onset atopy may be determined by 1 month of age. Low initial lung function in some individuals can 'recover' in some settings.