ABSTRACT. It is well recognized that delayed "first words" is among the most common presenting symptoms of autistic spectrum disorders (ASD). However, data on earlier language and communication development in children with ASD are limited to retrospective reports from parents and from home videos. In this study, we prospectively collected parent report data on early communication and language development in 97 infant siblings of children with ASD and 49 low-risk controls. Parents completed the MacArthur Communicative Development Inventory-Infant Form at 12 and 18 months. Analysis compared 3 groups defined on the basis of diagnostic assessment at 24 months: (1) siblings with ASD (n = 15), (2) siblings not meeting diagnostic criteria for ASD (n = 82), and (3) low-risk controls, none of whom had ASD (n = 49). Children with ASD showed delays in early language and communication compared with non-ASD siblings and controls. At 12 months, the ASD group was reported to understand significantly fewer phrases and to produce fewer gestures. At 18 months, they showed delays in their understanding of phrases, comprehension and production of single words, and use of gestures. Siblings not diagnosed with ASD also used fewer play-related gestures at 18 months than low-risk controls, even when children with identified language delays were excluded. Overall, this prospective study confirms that delays in communication and language development are apparent early in life in children with ASD, and emphasizes that developmental surveillance should include monitoring for delays in gesture, which may be among the earliest signs of ASD.