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.