There is a high incidence of behaviour disorder in people who are learning disabled. The purpose of this study is to assess whether risperidone is safe and effective in the treatment of learning disabled children diagnosed with behaviour disorder, which does not improve with psychological therapy. If the learning disability is very severe, or the behaviour disorder is very severe, such therapy is not appropriate and thus is not attempted; these patients are nevertheless eligible to participate in the study. At the beginning of treatment, the dose of risperidone is titrated, by gradual increase from 0.25 mg/day up to a maximum of 4 mg/day, based on therapeutic response, with decreases allowed in case of adverse reactions. Patients will be assessed over six months. The primary measure of effectiveness is the change compared to baseline in the total score on the Aberrant Behaviour Checklist (ABC). The EPSS is used to monitor the appearance of extrapyramidal symptoms. Based on results from other studies and the differences observed as a result of treatment, the investigator and his team expect to be able to detect a statistically and clinically significant result if data are available for 15 evaluable patients. Twenty patients will be recruited, taking account of possible drop outs. 0.25 mg of risperidone per day, taken orally, up to a maximum daily dose of 4 mg/day, for a treatment period of 6 months.