Recognizing early risk markers in young children with autism spectrum disorder (ASD) is critical for timely diagnosis and intervention. The purpose of this study was to extend previous findings regarding language milestones to a longitudinal design, in which ages of expressive language milestones (i.e., first words, first phrases) could serve as predictors of developmental trajectories in a heterogeneous sample of young children with ASD (N = 98; age at first assessment: M = 32 months, SD = 5). Age of first words predicted trajectories of expressive language and adaptive skills; number of words predicted each outcome examined. Because these aspects of early language show promise as potential indicators of later functional outcomes, future research on developmental processes as they relate to individual differences will be particularly informative.