Objective: Obsessive-compulsive disorder (OCD) is common in clozapine-treated patients alt-hough the actual prevalence, phenomenology and risk factors remain unclear. The aim of the pre-sent study was to address the three aforementioned questions. Method: The electronic records of a large cohort of clozapine-medicated schizophrenia patients routinely screened for OCD were used. The Obsessive Compulsive Inventory Revised version (OCI-R) was available from 118 cases and a 21 points cut-off threshold for OCD was defined. Results: OCD prevalence was 47%, higher in patients on poly-pharmacy than on monotherapy (64% vs 31%; p=.001). Two OCI-R factors had significantly higher scores and distinct risk factors: checking behaviour (mean=5.1; SD=3.6) correlated with length of clozapine treatment (r=.21; p=.026), and obsessing factor (mean=4.8; SD=3.6) correlated with psychosis severity (r=.59; p=.001). These factors along with total OCI-R, did not correlate with either clozapine dose or plas-ma levels, after correcting for psychosis severity. Conclusion: Screening for OCD in clozapine patients, and probably in those treated with structur-ally similar drugs with potent antiserotoninergic properties, should be widely adopted by clinicians. Further research is needed to understand the pathophysiology underlying repetitive behavior onset in clozapine-treated patients.