Objectives : there are two seriously limiting complications (agranulocytosis and seizure) in usage of clozapine as the major medication in treatment of schizophrenia. The present study was designed to investigate the degree of prevalence of seizure among patients under clozapine treatment. Methods: the subjects were all the patients under clozapine treatment in Shahid Esma'ili psychiatric center up to March 1998. Findings: the data collected from the files of 70 patients under clozapine treatment demonstrated that 9 patients (12.9%) were affected by some kind of seizure, of whom 6 (i.e. 8.6%) were affected by tonic-clonic seizure, and 3 (i.e. 43%) by myoclonic seizure. All the patients affected by seizure had received doses of 300 to 600 mg. of clozapine. Statistically, no meaningful correlation was found between appearance of seizure and variables such as sex, age, and dose of medication. Results: with regards to the rather high prevalence of seizure as demonstrated here, the study suggests greater care measures for prevention of such seizure among patients under clozapine treatment, such as avoiding simultaneous administration of multiple medications, and starting with low doses and gradually increasing the doses.