Objective : Chronic posttraumatic stress disorder (PTSD) is defined by the coexistence of symptoms (reexperiencing, avoidance and hyperarousal), persisting for more than months or years that causes significant impairment in social occupational functioning. This study was conducted to evaluate the effects of terazosin on posttraumatic nightmares which is one of chronic PTSD symptoms. Methods : Twenty patients with chronic PTSD and severe trauma-related nightmares were treated with terazosin (3-7 mg/day) for 8 weeks. Recurrent distressing dreams item of the Clinician administered PTSD Scale (CAPS), Total CAPS score, Clinical Global Impressions-Severity Scale (CGI-S) and Clinical Global Impressions-Change Scale (CGI-C) were performed at baseline, 4-week and 8-week. Mississippi Scale for Combat-Related PTSD and Combat Exposure Scale (CES) were used to evaluate PTSD symptom pattern and degree of exposure. Results : Significant decrease in recurrent distressing dreams item, reexperiencing, avoidance and hyperarousal symptom score and total CAPS score were at 4-week and 8-week treatment compared to baseline (recurrent distressing dreams item : $2.70{pm}1.88$ and $1.25{pm}1.49$ ; reexperiencing, avoidance and hyperarousal symptom score : $2.30{pm}2.49$, $1.20{pm}1.05$, $3.10{pm}1.68$ and $4.00{pm}3.59$, $1.55{pm}1.50$, $5.60{pm}3.23$ ; total CAPS score : $8.20{pm}6.26$ and $5.40{pm}5.89$). There were significant correlation between dosage of terazosin and improvement of PTSD symptoms. Adverse effects such as fatigue and orthostatic hypotension were showed, which were mild and self-limited. Conclusion : These results suggest that terazosin may improve severe trauma-related nightmares and overall PTSD symptoms. Randomized controlled study with more subjects would be necessary in the future.