The prevalence of a range of DSM-IV psychiatric disorders in a sample of 119 4–16-year-old children with Williams syndrome (WS) was assessed using a structured diagnostic interview with their parents. Most children (80.7%) met criteria for at least one DSM-IV diagnosis. The most prevalent diagnoses were Attention Deficit/Hyperactivity Disorder (ADHD; 64.7%) and Specific Phobia (53.8%). There was a significant shift in Predominant Type of ADHD as a function of CA, from Combined for the youngest group (ages 4–6 years) to Inattentive for the oldest group (ages 11–16 years). The prevalence of Generalized Anxiety Disorder (GAD) increased significantly with age. These findings are another step toward defining the behavioral phenotype of WS. © 2006 Wiley-Liss, Inc.