Increased aggressive behavior can occur in association with posttraumatic stress disorder (PTSD). This study tested the hypothesis that low-dose risperidone reduces aggression and other PTSD-related symptoms in combat veterans. Subjects were male combat veterans with PTSD who scored 20 or higher on cluster D (hyperarousal) of the Patient Checklist for PTSD-Military Version (PCL-M). Subjects were randomly assigned to either risperidone or placebo treatment groups. Drugs were administered over a 6-week treatment period in a double-blind manner. Subjects received either risperidone (0.5 mg/day; n = 7) or matched placebo (n = 8) tablets during the first 2 weeks of the treatment period. The dose of risperidone could then be increased up to 2.0 mg/day on the basis of response. Prerandomization psychotropic regimens were continued. Subjects were evaluated with the PCL-M and the Overt Aggression Scale-Modified for Outpatients (OAS-M). In comparison with placebo treatment, reductions in scores between baseline and the last week of treatment were significantly greater for OAS-M irritability and PCL-M cluster B (intrusive thoughts) subscales and on the PCL-M total scale. These results suggest that low-dose risperidone administration reduces irritability and intrusive thoughts in combat-related PTSD.