Objective: We conducted a two-period (open-label and double-blind) pilot investigation of droxidopa, with and without carbidopa, for ADHD. Method: Twenty adult ADHD patients received open-label droxidopa titrated from 200 to 600 mg 3 times per day (TID; Weeks 1-3), then open-label droxidopa plus carbidopa titrated from 25 or 50 mg TID (Weeks 4-6). In Weeks 7 to 8, patients were randomized to continued co-treatment or matching placebo substitution. Results: Improvements in mean total Adult ADHD Investigator Symptom Report Scale (AISRS) scores were seen at Week 1 (p < .0001) and Week 3 (p < .0001). Improvements were maintained but not increased with carbidopa. Thirteen of 20 patients completed open-label treatment. In the double-blind period, mean total AISRS scores were similar between the co-treatment (n = 6) and placebo (n = 5) groups. No serious adverse events were reported. Conclusion: These preliminary findings indicate that droxidopa can improve adult ADHD symptoms. Further studies are warranted to examine the efficacy and safety of droxidopa in ADHD.