Theophylline is well-established in the management of asthma, and there is some evidence of an antiinflammatory effect in asthma. It is not known whether theophylline affects inflammatory markers such as sputum eosinophils and exhaled nitric oxide (NO) in patients with mild asthma not receiving inhaled steroid therapy. In a double-blind, placebo-controlled, cross-over study of 15 patients with mild asthma, we assessed the effect of low-dose theophylline therapy (250 mg twice per day) on eosinophils in induced sputum, bronchoalveolar lavage (BAL) and airway biopsies at the end of both the treatment and placebo periods. Measurements of exhaled nitric oxide (NO) were made at the end of the active and placebo treatment periods of 5 wk each. Low-dose theophylline (mean serum level, 6.1 mg/L) led to a significant reduction in mean (95% confidence interval [CI]) sputum eosinophils from 11.3% (7.80-14.76%) to 8.0% (5.46-10.44%), BAL eosinophils from 3.4% (2.4-4.4%) to 1.7% (1.1-2.3%) and biopsy eosinophils from 1.83% (0.76-2.89%) to 1.20% (0.27-2.13%) compared with placebo (all p < 0.05). There was no significant change in levels of exhaled NO or improvement in lung function and bronchial responsiveness. Low-dose theophylline induced antiinflammatory effects in asthma, reflected by a fall in airway eosinophils with no change in exhaled NO or changes in lung function.