The present study evaluates the protective action of ketotifen on exercise-induced bronchospasm in patients diagnosed with bronchial asthma of different aetiologies. The patients were classified in 2 groups. The first group with seasonal asthma was made up of patients with pollen hypersensitivity, while the second group with perennial asthma was made up of patients with bacterial aetiology or sensitivity to the Dermatophagoides mite. Spirometry and airway resistance measurements (in basal conditions, 5 to 10 minutes after a resistance test which consisted of 6 minutes' free running on a treadmill) was performed on each patient. Those patients in which significant spirometry or Raw alterations were detected received a 15-day treatment of ketotifen (a dose of 1 mg every 12 hours); after this time, the tests mentioned before were repeated. We found that in the seasonal asthma group, ketotifen was effective in protecting against exercise. However, we did not observe the same effect in the perennial asthma group. In the light of these results, we propose the possible existence of different mechanisms in triggering off exercise-induced bronchospasm, according to their aetiologies.