Corticosteroids can have acute effects on airway function and methacholine airway responsiveness in asthma as early as 6 h after dosing, suggesting there may be an acute anti-inflammatory effect of inhaled corticosteroid in asthma. This study aimed to determine the effects of a single dose of inhaled budesonide on sputum eosinophils and mast cells in adults with asthma, and to examine whether the mechanism of clearance of eosinophils was by apoptosis. A randomized, double-blind, placebo-controlled, crossover study was conducted. At the screening visit, adults with stable asthma (n = 41) ceased inhaled corticosteroid therapy for 4 d and those with significant sputum eosinophilia (> or = 7%) were randomized (n = 26) to a single dose of budesonide 2,400 microg or placebo via Turbuhaler, on two separate study days. Symptoms and lung function were followed for 6 h, then sputum was induced and airway responsiveness to hypertonic saline determined. Sputum eosinophils (mean, SE) were significantly lower 6 h after budesonide (25%, 4.5), compared with placebo (37%, 6.2, p < 0.05). There was a 2.2-fold (95% CI 1.45 to 3.33) improvement in airway responsiveness with budesonide. No significant difference was seen on mast cells, apoptotic eosinophils, symptoms, or lung function. In conclusion, a single dose of inhaled corticosteroids has beneficial effects on airway inflammation and airway hyperresponsiveness as early as 6 h after dosing. This may be clinically useful as therapy during mild exacerbations of asthma.