Ketotifen in a dose of 1 mg p.o. q 12 h was compared with placebo in a double-blind, twelve month parallel group trial in 17 randomly selected patients with bronchial asthma. Among 13 patients treated with ketotifen, nine have been classified as responders and four as non-responders. Responders were young and teenaged individuals, athletically oriented, with moderate asthma. It is our impression that ketotifen offers a new therapeutic dimension in the prophylaxis of bronchial asthma.