Although much research has focused on the cognitive-linguistic profile associated with Williams syndrome, studies have yet to follow up on preliminary observations suggesting increased anxiety and fears in persons with this disorder. To this aim, Study 1 compared fears in 120 participants with Williams syndrome to 70 appropriately matched persons with mental retardation of mixed etiologies. Study 2 assessed differences in parent versus child reports of fears in 36 Williams syndrome and 24 comparison group parent-child dyads. In Study 3, rates of phobia and other anxiety disorders were assessed in standardized psychiatric interviews with the parents of 51 individuals with Williams syndrome. Relative to their counterparts, persons with Williams syndrome had significantly more fears as well as a wider range of frequently occurring fears, as reported by either parents or participants themselves. Children in both groups reported more fears than their parents. Whereas generalized and anticipatory anxiety were found in 51% to 60% of the sample with Williams syndrome, specific phobia was more prevalent, with 96% showing persistent and marked fears and 84% avoiding their fears or enduring them with distress. The feasibility of cognitive-behavioral treatments for phobia is discussed, as are implications for future research.