BACKGROUND: Based on an earlier pilot study, as well as a theoretical consideration of its mechanism of action, we undertook a placebo-controlled, double-blind trial of mirtazapine in posttraumatic stress disorder. METHODS: Twenty-nine patients were randomized to receive drug up to 45 mg/day or placebo double-blind on a 2:1 ratio for 8 weeks, with data being available for analysis in 26. Primary outcome measures comprised the Short Posttraumatic Stress Disorder Rating Interview (SPRINT) Global Improvement item and total score. Secondary measures comprised the Davidson Trauma Scale, Structured Interview for Posttraumatic Stress Disorder and Hospital Anxiety Depression Scale. Adverse events were also measured. RESULTS: On the Short Posttraumatic Stress Disorder Rating Interview Global Improvement measure, rates of response were 64.7% and 20.0% for mirtazapine and placebo. Treatment effects in favor of mirtazapine were noted on the Short Posttraumatic Stress Disorder Rating Interview global, Structured Interview for Posttraumatic Stress Disorder, and Hospital Anxiety Depression Scale anxiety subscale scores. The drug was well tolerated. CONCLUSIONS: Mirtazapine was more effective than placebo on some measures in posttraumatic stress disorder and general anxiety symptoms.