Objective: This study evaluates the efficacy, duration of action, and tolerability of methylphenidate transdermal system (MTS) in children with ADHD. Method: Participants were dose optimized over 5 weeks utilizing patch doses of 10, 16, 20, and 27 mg applied in the morning and worn for 9 hours. Following optimization, 80 participants were randomized to 1 week of MTS or placebo followed by 1 week of the opposite treatment. Laboratory classroom sessions conducted after each randomized week included blinded ratings of attention, behavior, and academic performance. Results: MTS was well tolerated and displayed significant improvement compared with placebo. Improvements were seen at the first postdose time point measured and continued through 12 hours. Conclusions: Treatment with MTS resulted in statistically significant improvements on all efficacy measures. Time course and therapeutic effects of MTS suggest that this novel methylphenidate delivery system is an efficacious once-daily treatment for ADHD. (Contains 6 tables and 3 figures.)