The clinical course of 210 children with perennial bronchial asthma was followed in a prospective controlled study in which one half of the children received placebo injections while the other half received conventional hyposensitization therapy. Of the 130 children still under observation at the time of their sixteenth birthday, 22% of the placebo-treated children were free of asthma compared to 72% of the treated children. In the treated group the rate of loss of asthma may be related to the dose of antigen received in hyposensitization therapy. Whereas 66% of the "1/5,000" group were free of asthma at the end of the study, 78% of the "highest tolerated dose" group were symptom free in their sixteenth year. The likelihood of a child outgrowing asthma was not significantly influenced by his sex, age of onset, or severity of his symptoms when first seen. A previous history of hay fever increased the likelihood of a child's asthma persisting into adolescence.