The aim of this double-blind cross-over study was to investigate whether treatment with the selective serotonin reuptake inhibitor, citalopram reduces aggressiveness in chronically violent schizophrenic inpatients. Initially 19 patients were enrolled into this double-blind cross-over study in which the patients were treated for 24 weeks with placebo and 24 weeks with citalopram (20-60 mg/day) as a supplement to their previous neuroleptic medication. Fourteen patients completed the entire study, but sufficient data on 15 patients could be used in the end-point analysis of efficacy. Psychiatric assessments (Brief Psychiatric Rating Scale, Clinical Global Impression Scale for Severity of Illness, Social Dysfunction and Aggression Scale and the Global Aggression Scale) and side effects (UKU Side Effect Scale) were recorded at baseline and 4 times during both periods. Aggressive incidents (Staff Observation Aggression Scale) were recorded throughout the study. During citalopram treatment, the frequency of aggressive incidents was significantly lower and the mental state did not deteriorate. Patients either experienced no side effects or else side effects were equally mild during both periods.