Non-word repetition, in which participants hear and repeat unfamiliar verbal stimuli, is thought to provide a particularly sensitive measure of verbal short-term memory capacity. However, performance on this task can also be constrained by hearing and speech production skills, and by an individuals' linguistic knowledge. This study examined real word and non-word repetition in a sample of 18 children and adolescents with Down syndrome, and compared their performance to that of a control group of 18 typically developing children in order to determine the extent of any impairment in repetition performance in Down syndrome, and the correlates of performance in each group. Individuals with Down syndrome were impaired in repetition performance, although both groups showed similar benefits of linguistic knowledge on repetition accuracy. An analysis of the correlates of non-word repetition revealed a double dissociation between the patterns of associations seen in the two groups. Among typically developing individuals, non-word repetition was related to an independent measure of verbal short-term memory but not to vocabulary knowledge. However, among individuals with Down syndrome, performance was related to vocabulary knowledge but not verbal short-term memory span. This contrast is consistent with the view that impairment to verbal short-term memory prevents this system from fully supporting non-word repetition in Down syndrome.