Williams syndrome (WS) is associated with cognitive deficits, special behavioral phenotype, and high rates of psychiatric disorders. The aims of the present study were: (1) To compare the rates of psychiatric disorders and repetitive behaviors in children with WS to children with idiopathic developmental disability (DDs); (2) To longitudinally assess the change in psychiatric disorders during adolescence in WS; (3) To assess retrospectively the effectiveness and safety of methylphenidate (MPH) treatment in WS children with ADHD. The study consisted of a cohort of 38 children and adolescents (age 13.1 ± 5.2 years) with WS and a sample of age-matched DDs (age 15.0 ± 3.1 years). A current follow-up evaluation was conducted after 5.6 ± 1.6 years for 25 subjects (65.8%) of the WS cohort. The rate of most psychiatric disorders was found similar in children with WS and DD controls. Specific phobia, especially from noises, obsessive–compulsive symptoms (e.g., aggressive obsessions and repetitive questions), and stereotypic behaviors (e.g., glancing), were more common in WS than DDs. In a longitudinal follow-up of the WS children, we found a decrease in the rate of anxiety disorders. In addition, a clinically significant improvement was reported in 72.2% of WS children with ADHD following MPH treatment. Sadness/unhappiness was the most common side effect associated with MPH treatment in WS, occurring in 2/3 of treated individuals. The present study further elucidates the neuropsychiatric phenotype of WS. Our results also suggest that MPH treatment for ADHD in WS warrants future prospective controlled trials. © 2011 Wiley Periodicals, Inc.