Objective: The authors evaluated the time course of the treatment effect of Osmotic-Release Oral System methylphenidate (OROS[R] MPH) HCl (Concerta[R], Raritan, NJ) CII in children with ADHD. Method: Data were combined from two double-blind, randomized, placebo-controlled, cross-over, analog classroom studies in children (9-12 years) with ADHD. Participants received an individualized dose of placebo or OROS[R] MPH on two laboratory school days. Permanent Product Math Test and Swanson, Kotkin, Agler, M-Flynn, and Pelham scores were evaluated 0.5 hr before dosing and 1, 2, 4, 10, 11, and 12.5 hr post dose. Analysis used a repeated-measures mixed model. Results: Treatment effects were present at all postdose assessment points (p less than 0.0001 for all comparisons, n = 139). Adverse events were similar to previous reports for OROS[R] MPH. Conclusion: A robust treatment effect occurred with OROS[R] MPH; onset was at 1 hr and persisted for at least 12.5 hr after dosing. (Contains 2 tables and 3 figures.)