Food intolerance seems to be an important cause of the hyperkinetic syndrome, but restricted diets are expensive, socially disruptive, and often nutritionally inadequate. Enzyme-potentiated desensitisation (EPD) may overcome some of these difficulties. EPD was tested in a double-blind placebo-controlled trial among 40 children with food-induced hyperkinetic behaviour disorder. A total of 185 children with established hyperkinetic syndrome underwent oligoantigenic dietary treatment for four weeks. 116 whose behaviour responded had provoking foods identified by sequential reintroduction. Foods that reproducibly provoked overactivity were avoided. 40 patients who were then invited to take part in the hyposensitisation trial were randomly assigned to treated and control groups. Treated patients received three doses of EPD (beta-glucuronidase and small quantities of food antigens) intradermally at two-monthly intervals. Controls received buffer only. Thereafter, patients were allowed to eat known provoking foods. Of 20 patients who received active treatment, 16 became tolerant towards provoking foods compared with 4 of 20 who received placebo (p less than 0.001). Our results show that EPD permits children with food-induced hyperkinetic syndrome to eat foods that had previously been identified as responsible for their symptoms. These results also support the notion that food allergy is a possible mechanism of the hyperkinetic syndrome.