Despite growing empirical evidence to the contrary, claims continue to be made that the grammar of people with Williams syndrome (WS) is intact. We show that even in a simple elicited imitation task examining the syntax of relative clauses, older children and adults with WS (n = 14, mean age = 17;0 years) only reach the level of typical five-year-old controls. When tested systematically in a number of different laboratories, all aspects of WS language show delay and/or deviance throughout development. We conclude that the grammatical abilities of people with WS should be described in terms of relative rather than absolute proficiency, and that the syndrome should no longer be used to bolster claims about the existence of independently functioning, innately specified modules in the human brain.