We investigated the effects of zuclopenthixol on aggressive behaviour in patients with intellectual disabilities by randomly withdrawing it after a 6-week period of open treatment. Of the 49 patients responding to the treatment, 39 took part in a randomised withdrawal trial. The placebo subgroup (n=20) showed more aggressive behaviour as indicated by outcomes observed by external raters on the Modified Overt Aggression Scale than did the continuing subgroup (n=19). The results indicate that discontinuation of zuclopenthixolin this population leads to an increase in aggressive behaviour.