Five groups of children defined by presence or absence of syntactic deficits and autism spectrum disorders (ASD) took vocabulary tests and provided sentences, definitions, and word associations. Children with ASD who were free of syntactic deficits demonstrated age-appropriate word knowledge. Children with ASD plus concomitant syntactic language impairments (ASDLI) performed similarly to peers with specific language impairment (SLI) and both demonstrated sparse lexicons characterized by partial word knowledge and immature knowledge of word-to-word relationships. This behavioral overlap speaks to the robustness of the syntax-lexicon interface and points to a similarity in the ASDLI and SLI phenotypes. (Contains 2 tables and 4 figures.)