This study investigates the syntactic abilities of ten individuals with Williams syndrome (WS) (mean chronological age: 8;9 years; mean mental age: 4;8 years) and Down's syndrome (DS) (mean chronological age: 8;7 years; mean mental age: 4;6 years), matched individually on chronological age, mental age and performance IQ. The syntactic components investigated include the comprehension of passives and the production, comprehension and repetition of wh-questions. Performance is compared to ten younger typically developing (TD) controls matched individually to both experimental groups on mental age (mean chronological age: 4;4 years; mean mental age: 5;0 years). Participants were given a standardized measure of grammatical ability and non-standardized tasks exploring the comprehension of active and passive sentences, and the production, comprehension and repetition of a range of wh-question types: wh-subject, wh-object, which NP-subject and which NP-object. Participants with WS and DS performed similarly on the standardized measure of grammatical ability, as well as on the experimental tasks that tapped comprehension of passives, and production and comprehension of wh-questions. Participants with DS performed significantly more poorly than both the WS cohort and TD controls on the repetition of wh-questions. Both the WS and DS cohorts performed significantly more poorly on most of the syntactic tasks compared to the younger TD controls. Individuals with WS and DS experienced significant difficulties in tasks measuring aspects of syntactic ability and performed more poorly than mental age-matched TD controls. Implications of these findings, with regards to the debates around language "intactness" in WS, as well as the similarities and differences in language abilities in WS and DS, dependent on age and developmental stages studied, are explored.