Currently, there is no systematic empirical information to guide the use of methylphenidate (a common brand name is Ritalin) to treat symptoms of ADHD in young children with Autistic Disorder/Asperger's Disorder/Pervasive Developmental Disorder, Not Otherwise Specified/Developmental Disorders. Preliminary data from a recent study of ADHD in young children suggest that methylphenidate may be useful in children with developmental disorders (DD). The purpose of this study is to determine the safety and efficacy of methylphenidate to treat ADHD symptoms in young children with Pervasive Developmental Disorders (PDD) or Developmental Disorders (DD).