The relationship between vocabulary and grammar in two children with 5p deletion syndrome

5p deletion syndrome is a rare genetic condition associated with severe speech and language problems. In general, research on speech and language skills is scarce, but there is more knowledge on phonetic and phonological skills than on lexical and grammatical skills. And till now no studies have addressed the relationship between grammar and vocabulary. Therefore, in this study, we address aspects of this relation based on longitudinal parent-reported data (MacArthur-Bates Communicative Development Inventories) from two children with this syndrome aged 2;0–7;3, and 1;11–7;1, respectively. We examine the development of the vocabulary size in each child, seen in relation to the development of grammar (inflections, combinations of words, complexity, and productivity), and see to what extent they can be compared to typically developing children. Results show that they follow a similar pattern to typically developing children but are delayed and have slightly different individual profiles.