Background: Sentence construction and syntactic organization are known to be poor in children with specific language impairments (SLI), but little is known about the way in which children with SLI approach language tasks, and static standardized tests contribute little to the differentiation of skills within the population of children with language impairments (LI). Information about the nature and intensity of prompts that facilitate sentence construction for a particular child may be useful in planning effective intervention. Aims: This paper describes the development of a dynamic assessment (DA) task which requires implicit knowledge of syntactic structure. The aim was to formulate a valid and reliable procedure for the DA of sentence formulation that could yield useful information for planning intervention for children with LI. Methods & Procedures: The Dynamic Assessment of Sentence Structure (DASS) was employed on 24 children aged 8-10 years, with identified language impairments, who were tested four times, at 4 monthly intervals. Outcomes & Results: A range of scores was elicited with no limiting ceiling or floor effects, and the test showed high internal reliability of alpha = 0.833. Inter-rater reliability was high. Concurrent validity was demonstrated by significant correlation with scores obtained on the CELF-3(UK) and predictive validity of the measure was also found to exceed that of the standardized test measure. Information about the ability of the children to use strategies and less directive prompts, and to transfer learning between items was elicited, and the information was thought to be useful by speech and language therapists involved in their management. Conclusions & Implications: The application of DA principles to the assessment of children previously diagnosed with LI, for the purposes of finding out more information about their potential to benefit from language intervention, was found to be effective. The tool developed was shown to be valid and reliable, and it has potentially important applications for the planning of individual intervention programmes and service delivery. (Contains 4 tables and 4 figures.)