Reduction in non-specific bronchial hyperreactivity has been reported in atopic asthmatic patients as a consequence of long-term treatment with sodium cromoglycate. As bronchial hyperreactivity is an undesirable feature in other forms of obstructive airways disease, we examined the effect of regular treatment with sodium cromoglycate (20 mg, 4 times daily, for 30 days) in a group of non-atopic bronchitic subjects who showed a significant bronchoconstrictor response to the inhalation of ultrasonically nebulised distilled water (fog challenge). After 30 days treatment with sodium cromoglycate, there was a significant reduction in response to fog challenge, compared with pretreatment values. There was a washout period of 12 h between the administration of the last dose of sodium cromoglycate and the second fog challenge. The possible mechanisms involved and the clinical significance of these findings are discussed.