To evaluate the prophylactic effect of ketotifen against the onset of asthma we selected 121 infants with atopic dermatitis, without any history suggestive of asthma (cough and/or wheezing). Sixty-one children received ketotifen twice daily. Those who weighed less than 14 kg received 0.8 mg; 14 kg or more, 1.2 mg. Sixty children, a placebo syrup indistinguishable from the active syrup. Both groups were followed for 1 year, with bimonthly evaluations. The criteria for onset of asthma were two different episodes of wheezing treated with bronchodilator drugs. Both groups were comparable regarding age, sex, weight, onset, and duration of atopic dermatitis and age at the onset of asthma. During the 1 year study, asthma was observed in eight children of the ketotifen group (13.1%) and in 25 children of the placebo group (41.6%) (P less than .001). Side effects were negligible and routine laboratory tests disclosed no significant alterations. Ketotifen is a very useful drug for prevention of asthma in children with atopic dermatitis and total IgE more than 50 IU/mL.