A controlled trial of the treatment of asthma by injections of a commercially produced extract of house dust is reported on 96 patients. The patients were allocated at random to two groups. One group received extract of house dust in a carbol saline solution, the other a carbol saline solution only. Some patients in both groups were advised on dust control in their houses. Results were available for comparison from 70 patients, 33 treated and 37 controls. Neither the treatment with house dust extract nor advice on dust control was found to give any advantage.