Ambroxol is a new compound which increases secretion of phosphatidylcholine by type II pneumocytes. The secretion of phosphatidylcholine could affect bronchial hyperreactivity in two ways: by increasing lysophosphatidylcholine turnover and/or by modifying the layer of bronchial secretion that covers airway receptors. To see whether ambroxol affects bronchial hyperreactivity, we carried out a double-blind cross-over study, evaluating the efficacy of this drug vs placebo in modifying methacholine PD20 in 11 asthmatic patients, 4 atopics and 7 non-atopics. 90 mg of ambroxol or placebo were randomly administered orally for one of two 14-day periods. Methacholine PD20 was evaluated before and after each treatment. A highly significant difference was demonstrated in the direct comparison between ambroxol and placebo as to their ability to modify mean metacholine PD20, which with ambroxol was more than double that with placebo. It was also seen that ambroxol induced a significant increase in baseline values of PD20, i.e. a significant decrease in bronchial hyperreactivity. Placebo did not do this.