We investigate whether children with Grammatical Specific Language Impairment (G-SLI) are also phonologically impaired and, if so, what the nature of that impairment is. We focus on the prosodic complexity of words, based on their syllabic and metrical (stress) structure, and investigate this using a novel non-word repetition procedure, the Test of Phonological Structure (TOPhS). Participants with G-SLI (aged 12-20 years) were compared to language-matched, typically developing children (aged 4-8 years). The results reveal that, in contrast to the controls, the accuracy with which the G-SLI group repeated non-words decreased as prosodic complexity increased, even in non-words with only one- and two-syllables. The study indicates that, in G-SLI, complexity deficits in morphology and syntax can extend to prosodic phonology. The study highlights the importance of taking into account prosodic complexity in phonological assessment and the design of non-word repetition procedures.