Medication phase: After screening assessments are completed, the child will enter the medication phase of the study. Each child will first have a step-wise single-blind titration of MPH to determine his/her best dose followed by a double-blind crossover trial with placebo and the child's best dose. The order of active drug and placebo will be randomized across the sample so that half the children will first receive MPH for 2 weeks and the other half will first receive placebo for 2 weeks. The children will receive the alternative drug condition (placebo or the child's best dose) for the next 2 weeks.