Objective: This study evaluates dexmethylphenidate extended release (d-MPH-ER) in adults with ADHD. Method: Following a 5-week, randomized, controlled, fixed-dose study of d-MPH-ER 20 to 40 mg/d, 170 adults entered a 6-month open-label extension (OLE) to assess long-term safety, with flexible dosing of 20 to 40 mg/d. Exploratory effectiveness outcomes included change from Week 5 on ADHD Rating Scale (ADHD-RS) and proportion of responders on Clinical Global Impressions-Improvement (CGI-I) scale. Results: 103 patients completed OLE, and effectiveness was evaluable in 102 patients. d-MPH-ER was well tolerated; the most common adverse events (greater than 15%) were headache, insomnia, and decreased appetite. Mean improvements in ADHD-RS score were -10.2 for patients switched from placebo to d-MPH-ER (n = 20) and -8.4 for those maintained on d-MPH-ER (n = 82). Respective CGI-I responder rates were 95.0% and 95.1%. Conclusion: Once-daily d-MPH-ER 20 to 40 mg is safe and effective for long-term treatment of adult ADHD. (Contains 4 tables and 1 figure.)