Objective: To evaluate the efficacy and safety of dexmethylphenidate hydrochloride (d-MPH, Focalin[TM]) for the treatment of attention-deficit/hyperactivity disorder (ADHD) and to test an a priori hypothesis that d-MPH would have a longer duration of action than d,l-threo-methylphenidate (d,l-MPH). Method: This was a randomized, double-blind study conducted at 12 U.S. centers. One hundred thirty-two subjects received d-MPH (n = 44), d,l-MPH (n = 46), or placebo (n = 42) twice daily for 4 weeks, with titration of the dose based on weekly clinic visits. The primary efficacy variable was change from baseline to last study visit on teacher-completed Swanson, Nolan, and Pelham Rating Scale (Teacher SNAP). Secondary efficacy measures included the change on parent-completed SNAP (Parent SNAP), Clinical Global Impressions Scale-Improvement (CGI-I) score, and Math Test performance. Assessments at home in late afternoon were included to test the hypothesis that d-MPH would have a longer duration of efficacy than d,l-MPH. Safety was assessed through monitoring occurrence and severity of adverse events and discontinuations related to them. Results: Treatment with either d-MPH (p = .0004) or d,l-MPH (p = .0042) significantly improved Teacher SNAP ratings compared with placebo. The d-MPH group showed significant improvements compared with placebo on the afternoon Parent SNAP ratings (p = .0003) and scores on the Math Test (p = .0236) obtained late in the afternoon at 6:00 P.M. Sixty-seven percent of patients showed improvement on d-MPH and 49% on d,l-MPH based on CGI-I scores. Both d-MPH and d,l-MPH were well tolerated, no patient in the d-MPH group and only two patients each in the d,l-MPH and placebo groups discontinued the study. Conclusions: For the treatment of ADHD, an average titrated dose of 18.25 mg/day of d-MPH is as efficacious and safe as an average titrated dose of 32.14 mg/day of d,l-MPH. Both active treatments have large effect sizes. Thus, d-MPH and d,l-MPH appear to provide similar efficacy, and d-MPH may have longer duration of action after twice-daily dosing, but additional studies are needed to determine the statistical and clinical significance of this possibility.