This will be a 12-week study of twenty enrolled subjects with DSM IV adult ADHD (age 18-55), with a goal of completing twenty subjects in the trial. The primary objective of this study is to determine the effect of droxidopa therapy on adult ADHD symptoms over the course of a six-week open-label titration period followed by a two-week double-blind, placebo-controlled period. The primary outcome measure will be changes from baseline in total score on the Adult ADHD Investigator Symptom Rating Scale (AISRS). Secondary measures will be changes in self-report ADHD symptoms on the ASRS v1.1 Symptom Checklist, global impairment on the Clinician Global Impression Scale (CGI).