 - Cardiovascular Risk Factors: Children with a personal history or family history of cardiovascular risk factors will be excluded, or given the option of participating in the study after obtaining an EKG and a signed letter from a pediatric cardiologist verifying the safety of their participation in a trial of methylphenidate. In this case, families will be responsible for the costs of EKG and cardiologist evaluation. If for any reason a family is unable to assume the cost of the EKG and cardiologist evaluations but still wishes for their child to participate, study staff will determine on a case-by-case basis whether the study budget allows the study to offer financial assistance to the families for these evaluations. 