Participants in this 14-week, double-blind, placebo-controlled study will be randomly assigned to receive either sustained-release MPH or placebo. All participants will receive individual cognitive behavioral therapy. The trial will last 14 weeks. It will include a 1-week placebo lead-in phase and a 2-week dose titration phase, followed by 11 weeks on a stable dose of MPH. During the titration phase, MPH will be given twice a day, starting at a dose of 10 mg/day. The dose will increase by 10 mg each day, until a final stable dose of 40 mg/day is reached. At this time, sustained-release MPH will be given as two 20 mg doses (one in the morning and one in the afternoon). Depending on a participant's tolerance of MPH, the dose will be increased to a maximum of 60 mg/day (40 mg in the morning and 20 mg in the afternoon). Participants who are unable to tolerate a dose of at least 40 mg/day of MPH will be discontinued from the study. Assessments of ADHD symptoms will be completed at weekly study visits. In addition, drug use assessments will also be completed and will include self-reports and urine toxicology tests.