The therapeutic effects of inhaled disodium cromoglycate (DSCG) and orally administered ketotifen were compared in thirty atopic asthmatics aged 15-34 years during a 22-week double-blind parallel group study. Ketotifen is a cycloheptathio-phene with experimental antihistaminic, anti-allergic and anti-anaphylactic effects equal or superior to those of DSCG. During the first 6 weeks of treatment, mean airflow meter readings increased and bronchodilator use diminished in those receiving DSCG, but no improvement was seen in those given ketotifen. In the next 10 weeks. concomitant therapy was reduced in both groups, but this reduction was greater in the group receiving DSCG. No serious adverse effects occurred. Asthma worsened after abrupt discontinuation of DSCG but not ketotifen. Although a small number of patients may have benefited from ketotifen. its effect on asthma was not comparable with that of inhaled disodium cromoglycate.