Background:Clozapine is a unique atypical antipsychotic medication. It is considered to be superior, even amongst the newer agents, in treatment-resistant schizophrenia. However, de novo emergence or exacerbation of obsessive-compulsive(OC) symptoms during treatment with clozapine has been reported. We prospectively evaluated 19 cases which newly developed OC symptoms during clozapine treatment and discussed the treatment of OC symptoms induced by it. Methods:We recruited 19 patients(8 males, 11 females) with a DSM-IV diagnosis of schizophrenia and schizoaffective disorder who had developed OC symptoms during clozapine treatment. OC symptoms were assessed using the Padua-ICMA and YBOCS on a monthly basis over three months. Results:Eleven female and eight male patients were enrolled and the average age of patients was 32.8 years. At baseline, no patients showed OC symptoms. Moderate to severe OC symptoms appeared with mean daily dose of 298.68 mg of clozapine. There were no significant differences in improving OC symptoms between the clozapine dose reduction group and the OC treatment group. Conclusion:We noticed the possibility that the appearance of OC symptoms is connected with the effect of clozapine. The clozapine-induced OC symptoms were improved both by reducing clozapine daily doses, and by adding OC treatment drugs. With other atypical antipsychotics now available, to know and treat the side effects of clozapine would be of considerable value, offering clinical guidance in making a decision on treatment-resistant schizophrenia.