Aim: This study investigated whether components of attention and executive functioning improve when children with attention-deficit-hyperactivity disorder (ADHD) are treated with osmotic-release oral system (OROS) methylphenidate. Method: Thirty children (24 males, six females; mean age 8y 6mo, SD 1y 11mo; range 6y 5mo-12y 6mo) with ADHD combined type participated in a double-blind, placebo-controlled crossover trial with the child's clinically most effective dose as identified with a systematic open-label titration procedure. After 1 week on each treatment (placebo and OROS methylphenidate), a neuropsychological battery that assessed sustained attention, selective attention, attentional control, response inhibition, and working memory was administered. This battery included the Gordon Diagnostic System, seven subtests of the Test of Everyday Attention for Children, and two tests of working memory. Results: Performance on two of three tests of response inhibition improved on OROS methylphenidate compared with placebo (p less than 0.01). Performance on one of two tasks assessing attentional control and one of five measures assessing sustained attention demonstrated clear improvement. There was no improvement on the two tasks assessing selective attention or the two tasks assessing working memory. Interpretation: When OROS methylphenidate was used to treat children with ADHD at the clinically most effective dose, general improvement was noted on tasks requiring response inhibition; response to treatment in other domains was either variable or not demonstrated.