To determine whether inhaled frusemide, a diuretic able to interfere with ion and water movement across airway epithelium, can modify exercise-induced bronchoconstriction, a three-part randomised, double-blind, placebo-controlled study was done in asthmatic patients who had a fall in FEV1 of at least 20% after running up and down a corridor. In the first part the effect of approximately 28 mg frusemide given as an aerosol was compared with that of a placebo. In the second part two doses of inhaled frusemide (approximately 14 mg and 28 mg) were examined. In the third part the effect of 20 mg oral frusemide was tested. Inhaled frusemide had a good and dose-related protective effect, whereas oral frusemide was ineffective. The mean (95% CI) maximum percentage falls in the FEV1 were: 11.5 (14.3-8.7) with frusemide and 33.8 (39.1-28.5) with placebo in the first part of the study; 13.6 (21.6-6.0) with 28 mg frusemide, 19.7 (28.2-11.3) with 14 mg frusemide, and 34.6 (39.4-30.0) with placebo in the second part of the study; and 15.2 (19.9-10.5) with inhaled frusemide, 38.2 (47.1-29.3) with oral frusemide, and 35.3 (45.9-24.7) with placebo in the last part of the study. The findings lend support to the hyperosmolarity hypothesis of exercise-induced asthma and may have therapeutic implications.