Language in autism has been the subject of intense interest, because communication deficits are central to the disorder, and because autism serves as an arena for testing theories of language acquisition. High-functioning older children with autism are often considered to have intact grammatical abilities, despite pragmatic impairments. Given the heterogeneity in language skills at younger ages, this assumption merits further investigation. Participants with autism (n = 21, aged nine to seventeen years), matched on chronological age, receptive vocabulary and IQ, to 22 typically developing individuals, completed a grammaticality judgment task. Participants with autism were significantly less sensitive than controls, specifically for third person singular and present progressive marking. Performance interacted with sentence length, with lower sensitivity to errors occurring at the end of the longest stimulus sentences. Performance sensitivity was associated with onset of single word and phrase speech, and with severity of autistic symptomatology. Implications of findings are discussed.