Objective: Centrally active alpha-l-adrenergic-receptor antagonists such as prazosin are effective in the treatment of nightmares in patients with posttraumatic stress disorder (PTSD). A pharmacological alternative is doxazosin, which has a longer half-life and fewer side effects. However, doxazosin is currently being used without solid empirical evidence. Furthermore, no study so far has assessed the effects of alpha-l-antagonists on nightmares in patients with borderline personality disorder (BPD). We retrospectively assessed the effectiveness of doxazosin on nightmares in PTSD and BPD. Method: A retrospective chart review of patients treated with doxazosin for trauma-associated nightmares in our clinic was performed. As in previous prazosin studies, the B2 score of the Clinician-Administered PTSD Scale (CAPS) was used as the primary outcome measure. Furthermore, the Pittsburgh Sleep Quality Index-Addendum for PTSD (PSQI-A) and sleep logs were analyzed. Results: We identified 51 patients with PTSD and/or BPD (mean age 35.7 years, 92.3 % women) who received doxazosin for nightmares. Of these, 46 patients continued doxazosin over a 4-week period and 31 patients over a 12-week period. Within the 12-week period, doxazosin treatment significantly reduced nightmares regardless of PTSD/BPD. 25 percent of patients treated for 12 weeks had full remission of nightmares. PSQI-A scores indicated that additional trauma-associated sleep symptoms improved over 12 weeks. Furthermore, recuperation of sleep improved with doxazosin within the first 4 weeks of treatment. Conclusion: Doxazosin might improve trauma associated nightmares and more general sleep parameters in patients with PTSD and BPD. Randomized controlled trials are warranted.