Although previous research and clinical observation have indicated that individuals with Williams syndrome have a distinctive personality, an empirically derived personality profile has not been developed. The objective of the current investigation was to develop a personality profile that is descriptive of and distinctive to children with Williams syndrome. Participants were 23 8- to 10-year-old children with Williams syndrome and 20 8- to 10-year-old children with developmental disabilities of other etiologies. Participant groups had equivalent intellectual abilities. Parents completed measures of childhood temperament (Children's Behavior Questionnaire [CBQ]) and personality (parent report, short form of Multidimensional Personality Questionnaire [MPQ]). Using group comparisons and signal detection theory, we contrasted the personality characteristics of children with Williams syndrome and children with developmental disabilities of other etiologies. On the CBQ, high mean ratings on shyness (reverse-coded) and empathy together characterized 96% of the children in the Williams syndrome group, but only 15% of the mixed etiology group. On the MPQ, high ratings on items measuring certain characteristics combined (gregarious, people-oriented, tense, sensitive, and visible) were characteristic of 96% of the Williams syndrome group but only 15% of the mixed etiology group. The personality profiles emerging from the CBQ and MPQ provide a crucial step toward investigations of genotype/phenotype relations.