Objective: To compare the effects of methylphenidate on attention-deficit/hyperactivity disorder (ADHD) subtypes. Method: Nineteen ADHD/inattentive (ADHD/I) and 22 ADHD/combined (ADHD/C) 6- to 12-year-old children entered a 6-week, double-blind trial of placebo and methylphenidate in divided doses (0.94 [plus or minus] 0.02 mg/kg/day = 33.06 [plus or minus] 1.40 mg/day). ADHD children received a restricted arithmetic task without medication before the trial and after their noon dose on the last day of each phase. Thirty-four unmedicated controls were tested at comparable time points. Parents and teachers rated ADHD children before and after each phase of the trial; parents rated controls before the study. Results: Controls had marginally better arithmetic performance than children with ADHD/C who outperformed ADHD/I children. Unmedicated children with ADHD exceeded controls in task-incompatible behaviors during restricted arithmetic. Under methylphenidate, both ADHD subtypes reached control levels of arithmetic performance and task-incompatible behavior. Before the trial, parents rated children with both ADHD subtypes higher than controls on inattention, hyperactivity, and oppositionality/aggression and parents and teachers rated ADHD/C children higher than ADHD/I children on hyperactivity and oppositionality/aggression but not inattention. Methylphenidate lowered parent and teacher ratings of inattention and hyperactivity for those with both ADHD subtypes, but ratings of children with ADHD/C decreased more in hyperactivity and aggression. Conclusions: Methylphenidate ameliorated task-incompatible behavior, arithmetic performance, and inattention comparably in both ADHD subtypes, whereas medication reduced hyperactivity and aggression largely in children with ADHD/C. (Contains 4 tables.)