Questionnaires completed by the caregivers of 119 adults with Williams syndrome or idiopathic infantile hypercalcaemia gave information on current living arrangements, daytime occupations and attainments, self-care skills, social relationships, behavioural and adjustment difficulties and the impact of Williams syndrome and idiopathic infantile hypercalcaemia on caregivers and their families. Many were reported to have marked behavioural and social difficulties persisting into adulthood, and most required some supervision and support in everyday activities such as dressing, toileting and preparing food. Some caregivers were dissatisfied with the level of educational attainment, current daytime occupations and support received from professionals. Over half were concerned about future living arrangements and daytime occupations. Comparisons between the questionnaires for the two groups revealed no phenotypical differences in their abilities or behavioural characteristics.