BACKGROUND A recent double blind clinical trial in Japan has shown that auranofin (6 mg/day) is a useful treatment for patients with moderate to severe asthma. To investigate the mechanism of action of auranofin the bronchial responsiveness to inhaled methacholine has been studied in well controlled asthmatic subjects. METHODS Nineteen adult asymptomatic asthmatic subjects received auranofin (3 mg orally twice a day) or inactive placebo in random order for 12 weeks in a double blind fashion. Bronchial responsiveness to inhaled methacholine and pulmonary function tests were measured at the same time on different days before, and six and 12 weeks after, each treatment. RESULTS Non-specific bronchial hyperresponsiveness 12 weeks after treatment with auranofin was decreased compared with that before treatment with auranofin and 12 weeks after treatment with inactive placebo, although the treatment did not improve pulmonary function tests. CONCLUSIONS Non-specific bronchial hyperresponsiveness 12 weeks after treatment with auranofin is decreased in a group of mild asymptomatic asthmatic patients with normal lung function.