A double-blind placebo-controlled cross-over trial was carried out to evaluate the efficacy and safety of the combined serotonin-dopamine antagonist risperidone in mentally retarded patients with persistent behavioural disturbances. After an observation period of 1 week, risperidone 4-12 mg or placebo was administered during 3 weeks as add-on treatment to the existing medication, followed by a 1-week single-blind placebo wash-out, and another 3 weeks of double-blind treatment with the cross-over medication. Thirty-seven patients participated in the trials; 30 completed the study. Risperidone was significantly superior to placebo in its effect on the Aberrant Behaviour Checklist and the Clinical Global Impression. The Extrapyramidal Symptom Rating Scale did not show any differences between risperidone and placebo. Two patients experienced hypotension at the start of the risperidone administration. Sedation and drowsiness were the most frequently reported treatment-emergent adverse events. The results of this trial warrant further investigation into the therapeutic assets of risperidone in this indication, as add-on therapy and as monotherapy.