The association between schizophrenia and obsessive-compulsive disorder (OCD) is complex. This study systematically examined a UK cohort of clozapine-treated individuals with schizophrenia/schizoaffective disorder. Fourteen of 59 cases (24%) scored positively on item H of the Mini-International Neuropsychiatric Interview (MINI) for OCD. The mean Yale- Brown Obsessive-Compulsive Scale (Y-BOCS) score in MINI-positive cases was 17.6 (SD+/-6.3). Sixty-four percent scored 16 or more on the Y-BOCS, representing clinically meaningful illness severity. Seven (50%) patients with OCD had previously received the diagnosis by their treating clinicians and were already receiving with selective serotonin re-uptake inhibitors (SSRIs) treatment. OCD cases scored significantly worse than their non-OCD counterparts on the Abnormal Involuntary Movement Scale (P=0.01) and the Simpson Angus Scale (SAS; P=0.01). There was also a non-significant trend toward higher ratings for OCD cases on the Clinical Global Impression-Schizophrenia scale (P=0.06). Comparing the OCD cases taking SSRI (n=7) with those not on SSRI (n=7), significant differences emerged on the SAS (P=0.03). Our results suggest that OCD is common among patients receiving clozapine for schizophrenic disorders and that the comorbidity is associated with greater motoric impairment. The role of medication in this condition remains unclear.