Behavioral and emotional disturbance was assessed in 70 children and adolescents with Williams syndrome. They were compared with an epidemiological control population, which was statistically controlled for age, gender, and level of mental retardation. Those with Williams syndrome were more likely to be diagnosed as suffering psychiatric disorder. The disorder was characterized by anxiety, hyperactivity, preoccupations, and inappropriate interpersonal relating. Significantly increased rates of other individual symptoms were also found, including sleep disturbance and hyperacusis. These results, considered with earlier findings, suggest that there is a valid behavior phenotype of Williams syndrome. This is frequently associated with sufficient impairment to consider inclusion of the behavior phenotype in future official taxonomies of mental disorders.