BACKGROUND Results from open-label and placebo-controlled trials suggest that the selective serotonin reuptake inhibitors reduce impulsive aggressive behavior. The objective of this open-label study was to investigate whether citalopram treatment has anti-aggressive effect on impulsive aggressive subjects meeting DSM-IV criteria for a cluster B personality disorder or intermittent explosive disorder. METHODS In this 8-week trial, subjects were initiated on 20 mg/day of citalopram and titrated up to 60 mg/day by the fourth week, if tolerated. The primary outcome measure was the Overt Aggression Scale-Modified (OAS-M), a scale used to quantify verbal and physical aggression, subjective irritability, and overt irritability. Secondary outcome measures included the Barratt Impulsiveness Scale and Buss-Durkee Hostility Inventory. RESULTS Of 25 subjects enrolled, 20 completed the study. The mean daily dose was 45.5 mg, and citalopram was generally well tolerated. Statistically significant decreases were found in the OAS-M aggression scores (32.82 +/- 19.76 to 4.73 +/- 7.57, p =.000), subjective irritability scores (3.50 +/- 0.60 to 1.45 +/- 1.18, p =.000), and overt irritability scores (3.23 +/- 0.81 to 0.91 +/- 1.02, p =.000). CONCLUSIONS These results suggest that citalopram is an effective treatment for reducing impulsive aggressive behavior.