Several studies have shown that long-term administration of inhaled corticosteroid reduces airway hyperresponsiveness. This study was performed in order to exclude an acute effect of inhaled corticosteroid. In a double-blind, randomized, cross-over study, children with asthma, who had never used inhaled or oral corticosteroid, received a single dose of 0.8 mg budesonide or placebo on two separate days, with an interval of at least 48 h. On each test day, baseline forced expiratory volume in one second (FEV1) and methacholine responsiveness (expressed as provocative dose producing a 20% fall in FEV1 (PD20) to methacholine, in doubling dose) were measured. Both measurements were repeated 2 and 5 h after administration of the drug. Twenty children were included in the study. FEV1 showed a mean increase of 1% at 5 h on the budesonide day, and a decrease of 2% on the placebo day (p = 0.01). PD20 increased by 0.1 doubling dose on the budesonide day, and decreased by 0.4 doubling dose on the placebo day. These changes are within the measurement variation (p = 0.06). We conclude that a single dose of 0.8 mg budesonide has a minor effect on methacholine responsiveness 5 h after administration in children with asthma. It is unlikely that such an effect interferes with the interpretation of data collected in long-term studies.