Clinical data and uncontrolled observations have suggested that fluoxetine is helpful in some patients with borderline personality disorder. This article describes the results of a 13-week double-blind study of volunteer subjects with mild to moderately severe borderline personality disorder. Thirteen fluoxetine recipients and nine placebo recipients received treatment. Pretreatment and posttreatment measures were obtained for global mood and functioning, anger, and depression. The most striking finding from this study was a clinically and statistically significant decrease in anger among the fluoxetine recipients. This decrease was independent of changes in depression. These data support previous observations that fluoxetine may reduce anger in patients with borderline personality disorder. The number of subjects in this study was small, the placebo responsiveness was high, and the clinical characteristics of the patients were in the mild to moderate range of severity. The data cannot be extrapolated to more severely ill borderline patients, but further study of fluoxetine and other selective serotonin reuptake inhibitors is indicated in this population.