To investigate the relationship between language acquisition and cognition, we evaluated linguistic abilities in 12 Italian-speaking children with Williams syndrome (WS) and 12 with Down syndrome (DS) of comparable global cognitive level. Another control group included 12 typically developing (TD) children, matched for mental age. Linguistic measures included a parent questionnaire to assess vocabulary, a verbal comprehension test, a sentence repetition test and MLU calculated on spontaneous production. No dissociation was evident between lexical and cognitive abilities, but specific morphosyntactic difficulties emerged both in comprehension and production in children with DS. Individuals with WS, albeit less compromised than DS, also had difficulty in the phrase repetition task and, particularly, using content words. Our results demonstrate that the linguistic abilities of infants with WS are not above their cognitive level and that language development in these special populations is not only delayed, but follows a different developmental trajectory.