Summary We studied 17 patients in a double-blind controlled trial to see if ketotifen, a new oral mast-cell stabilizing agent, would protect against bronchoconstriction induced by exercise on a treadmill. In addition eight of the patients also completed exercise tests without previous medication and again after inhalation of disodium cromoglycate (DSCG). The greatest amount of bronchoconstriction was caused by exercise without preliminary treatment. There was less bronchoconstriction after ketotifen, but this was no better than a placebo. DSCG provided significant protection. We were unable to confirm previous reports that ketotifen is as effective as DSCG in preventing exercise-induced asthma.