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The purpose of this study was to compare the efficacy and safety of extended-release dexmethylphenidate (d-MPH-ER) to that of d,l-MPH-ER and placebo in children with attention-deficit/hyperactivity disorder (ADHD) in a laboratory classroom setting. This multicenter, double-blind, crossover study randomized 82 children, 6 to 12 years of age, stabilized on a total daily dose to the nearest equivalent of 40 to 60 mg of d,l-MPH or 20 or 30 mg/day of d-MPH. Patients participated in a screening day and practice day, and were randomized to 1 of 10 sequences of all five treatments in five separate periods. Treatments included d-MPH-ER (20 mg/day), d-MPH-ER (30 mg/day), d,l-MPH-ER (36 mg/day), d,l-MPH-ER (54 mg/day), and placebo. Primary efficacy was measured by the change from predose on the Swanson, Kotkin, Agler, M-Flynn, and Pelham (SKAMP) Rating Scale-Combined scores at 2-h postdose during the 12-h laboratory assessment (d-MPH-ER 20 mg/day vs. d,l-MPH-ER 36 mg/day). Adverse events were monitored throughout the study period. d-MPH-ER (20 mg/day) was significantly more effective than d,l-MPH-ER (36 mg/day) in the primary efficacy variable, change from predose to 2-h postdose in SKAMP-combined score. In general, d-MPH-ER had an earlier onset of action than d,l-MPH-ER, while d,l-MPH-ER had a stronger effect at 12-h postdose. No serious adverse events were reported. Treatment with either agent was associated with significant improvements in ADHD symptoms. d-MPH-ER and d,l-MPH-ER can be differentiated on what part of the day each is more effective.