Children as young as 6 years old can perform spirometry, yet the relationship between current asthma, lung function, and bronchial responsiveness has not been described at this age; 2,537 children from a community-based birth cohort were assessed at 6 years of age, with history (n = 2,141), physical examination (n = 1,995), standard spirometry (n = 1,735), and a random sample (n = 711) offered methacholine challenge. Males had greater values of FVC and FEV(1) but not of mean forced expiratory flow during the middle half of the FVC or FEV(1)/FVC than females. The greatest influences on lung function at 6 years were height, sex, birth weight, and wheezing in the first year of life. Children with current asthma had small but significant deficits in lung function and were more sensitive to methacholine. The optimal cutpoint for determining heightened bronchial responsiveness was found to be a 15% fall in FEV(1) at a dose of 1.8 mg/ml. A negative test could be useful in excluding a diagnosis of asthma (negative predictive value of 92%). Lung function testing, including methacholine challenge, is feasible in 5- to 7-year-old children and has the potential to contribute to the clinical management of children with asthma.