This study investigates syntax in Autism Spectrum Disorders (ASD), its parallelism with Specific Language Impairment (SLI) and its relation to other aspects of cognition. We focus on (1) 3rd person accusative clitic (ACC3) production, a clinical marker of SLI hypothesized to relate to WM, and (2) 1st person accusative clitic (ACC1) production, preserved in SLI but hypothesized to be affected in ASD due to Theory-of-Mind (ToM) difficulties. Participants included 21 individuals with ASD (aged 5-16), 22 individuals with SLI (aged 5-16), age-matched and younger TD controls (N = 44). Clinical groups showed similar deficits for ACC3 and general morphosyntax. Closer analysis revealed that a subgroup of children with ASD displayed intact grammar except for ACC1, where children with SLI performed well. Better ToM scores implied better ACC1 scores in ASD. Difficulties with WM emerged for ASD and SLI and correlated only with performance on ACC3. Non-verbal reasoning was unrelated to syntactic measures.