The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition borderline personality disorder (BPD) seems to constitute a very heterogeneous category. Therefore, pharmacological therapy is symptom-oriented or targets comorbid conditions. A high comorbidity exists between BPD and posttraumatic stress disorder (PTSD). In a double-blind, randomized, placebo-controlled crossover study, we sought to determine whether the antinoradrenergic agent clonidine was effective in reducing hyperarousal and measures of BPD-specific and general psychopathology in a sample of 18 patients with BPD, with or without comorbid PTSD, and with a prominent hyperarousal syndrome. Hyperarousal as measured by the Clinician Administered PTSD scale improved significantly compared with placebo (P = 0.003) irrespective of PTSD comorbidity. Improvements in general and BPD-typical psychopathology were mainly seen in the PTSD-positive subgroup, whereas the subjective sleep latency (P = 0.005) and the restorative qualities of the sleep (P = 0.014) improved in the whole sample. Improvements, despite the small sample size of this pilot study, lead us to conclude that clonidine might be a useful adjunct to pharmacotherapy in patients with BPD who have marked hyperarousal and/or sleep problems and, in particular, in patients with BPD who have a PTSD comorbidity.