Risperidone is a novel and atypical agent with a dual antagonistic effect on 5-HT and D receptors. Open-label reports and one controlled study suggest that risperidone addition is effective in patients with obsessive-compulsive disorder refractory to treatment with serotonin reuptake inhibitors. However, risperidone has also been implicated in the production or exacerbation of obsessive-compulsive symptoms. We report six cases (schizophrenia, five cases; psychotic depression, one case) in which risperidone was effective in the treatment of the psychotic symptoms but produced obsessive-compulsive symptoms (four cases) or caused exacerbation of previous obsessive-compulsive symptoms (two cases). In all but one case, obsessive-compulsive symptoms emerged shortly after initiation of risperidone treatment with a dose above 3 mg/day. The mechanisms and risk factors for risperidone and other atypical antipsychotics to induce or exacerbate obsessive-compulsive symptoms are as yet not clear. Risperidone-induced obsessive-compulsive symptoms appear to be dose-dependent and are probably produced by serotoninergic-dopaminergic imbalance. Close monitoring of the patients receiving risperidone, especially those vulnerable to the development of obsessive-compulsive symptoms, may be of value. Gradual escalation and low final dose may be helpful.(2) (2)