Clozapine is an antipsychotic with superior efficacy in treatment refractory patients, and has unique anti-suicidal properties and a low propensity to cause extrapyramidal side-effects. Despite these advantages, clozapine utilization is low. This can in part be explained by a number of potentially lethal side effects of clozapine. Next to psychiatrists nurses play a crucial role in the long-term management of patients with schizophrenia. It is therefore important that nurses know, inform and monitor patients about the specific side-effects of clozapine. A recent study of psychiatrists published in 2011 has shown that there was a gap in the knowledge about side-effects of clozapine. The knowledge about side-effects of clozapine in nurses has never been studied. This cross-sectional study evaluated the knowledge base regarding the safety of clozapine, and its potential mediators, of psychiatric nurses in 3 psychiatric hospitals in Belgium with a specifically developed questionnaire based on the literature and expert opinion (3 clozapine experts). A total of 85 nurses completed the questionnaire. The mean total score was 6.1 of a potential maximum score of 18. Only 3 of the 18 multiple choice knowledge questions were answered correctly by more than 50% of nurses. Only 24.9% of participants passed the test (> 50% correct answers). Nurses working on psychosis units were more likely to pass the test (xx.y% vs yy.z%, p = 0.0124). There was a trend that nurses with a lower nursing diploma were more likely to fail the test (p = 0.0561). Our study clearly identifies a large gap in the basic knowledge of psychiatric nurses about clozapine and its side-effects. Knowledge could be increased by more emphasis on the topic in nurse's training curricula as well as targeted onsite training. Only 23.5% of participants indicate that there was sufficient information in their basic nursing training.