Adverse effects related to clozapine were assessed within a post-marketing drug surveillance program, the AMUP study, in two university psychiatric departments. In a randomly selected sample of patients (intensive drug monitoring) ADRs of any type were observed in 76% of clozapine-treated inpatients. Sedation, hypersalivation, increase in transaminases, and EEG changes were most frequently observed, but only rarely required changes in therapy. In 8.1% of 959 patients exposed to clozapine in the total inpatient population of the participating hospitals ADR led to withdrawal of clozapine; in 3.9% reactions judged as severe and potentially life-threatening occurred. Among these latter toxic delirium prevailed. In addition, four cases of severe cardiovascular and respiratory dysregulation were observed with the combination of clozapine and benzodiazepines. These cases and one case of sudden death under clozapine and haloperidol treatment are presented in some detail. The results obtained for clozapine are compared to data from this drug surveillance program for other neuroleptics.