Factors that might influence lung function bronchodilator response by 2 yr of age is largely unknown, thus we aimed to assess this in the 'Environment and Childhood asthma' (ECA) study in Oslo. A clinical investigation at mean age 26 months was attended by 516 (84%) children included in a nested case-control study [children with recurrent bronchial obstruction (rBO)] (n = 265) and controls without a history of lower respiratory disease (n = 251). Tidal lung function measures before and after inhaled nebulized salbutamol (bronchodilator response) (when clinically without BO) were obtained in 46%. Clinical characteristics and personal and family history of allergic/respiratory diseases (asthma risk factors) were ascertained by structured interview and clinical examination. Allergic sensitization was assessed by skin prick test/specific IgE antibody analyses to common allergens. Mean (95% CI) per cent change in time to reach peak flow/total expiratory time (t(PTEF)/t(E)) from before to after salbutamol was significantly larger in children with rBO [17.3 (9.4-25.3) than controls (-2.2 (-7.7 to 3.0)]. The bronchodilator response increased significantly (p = 0.001) with increasing number of asthma risk factors, but was not significantly associated with allergic sensitization, parental 'atopy', or maternal smoking alone. Children treated with inhaled corticosteroids had greater bronchodilator response than those treated with bronchodilators alone. Bronchodilator response in asymptomatic 2-yr-old children was most closely associated with the presence of rBO, but increasing number of asthma risk factors and treatment with inhaled corticosteroids were associated with increased bronchodilator response.