Twenty asthmatic children with laboratory proved bronchial reactivity to rye-grass pollen were studied over two consecutive grass-pollen seasons. In the first year 11 patients received preseasonal hyposensitisation treatment with an aqueous rye extract and nine received placebo injections. No treatment was given in the second year. Patients in both the active-treatment and placebo groups showed a pronounced clinical deterioration in their asthma during both pollen seasons. Serum concentrations of IgG-specific antibodies to the rye allergen before treatment were similar in both groups, but after immunotherapy and before the pollen season in the first year these antibody concentrations were raised significantly in the treated group (p less than 0.005): by the middle of the pollen seasons the difference was no longer significant. IgE-specific antibodies showed a similar but nonsignificant pattern of response. We found no evidence that limited hyposensitisation with a pollen extract is of any clinical benefit in seasonal asthma despite evidence of an immunological response.