This study describes the profile of repetitive behaviour in individuals with Williams syndrome, utilising cross-syndrome comparisons with people with Prader–Willi and Down syndromes. The Repetitive Behaviour Questionnaire was administered to caregivers of adults with Williams (n = 96), Prader–Willi (n = 103) and Down (n = 78) syndromes. There were few group differences, although participants with Williams syndrome were more likely to show body stereotypies. Individuals with Williams syndrome also showed more hoarding and less tidying behaviours than those with Down syndrome. IQ and adaptive ability were negatively associated with repetitive questioning in people with Williams syndrome. The profile of repetitive behaviour amongst individuals with Williams syndrome was similar to the comparison syndromes. The cognitive mechanisms underlying these behaviours in genetic syndromes warrant further investigation.