Ketotifen is a new chemical entity which has been shown pharmacologically to be both an anti-histamine and an anti-allergic agent. The oral activity of the compound has been investigated in a controlled study against placebo in 40 atopic and 38 non-atopic asthmatics. In atopic patients, ketotifen resulted in an improvement in asthma symptoms over the 12 week trial period. A significant number of patients on ketotifen were able to reduce concomitant bronchodilator therapy; there were no changes in the placebo group. The results from the non-atopic patients showed that there was no improvement on ketotifen in either asthma symptoms of lung function tests. The atopic patients who had symptoms of rhinitis showed significant improvement on ketotifen over the treatment period. There was no laboratory evidence of any untoward effect of ketotifen. Thus ketotifen showed a benefit in treating atopic asthmatic subjects. Non-atopic subjects showed no response. This differential efficacy parallels the effect of ketotifen on immediate immune responses.