Retrospective studies indicate 2 major classes of autism spectrum disorder (ASD) onset: early and later, after a period of relatively healthy development. This prospective, longitudinal study examined social, language, and motor trajectories in 235 children with and without a sibling with autism, ages 6-36 months. Children were grouped as: ASD identified by 14 months, ASD identified after 14 months, and no ASD. Despite groups' initial similar developmental level at 6 months, ASD groups exhibited atypical trajectories thereafter. Impairment from 14 to 24 months was greater in the Early-ASD than the Later-ASD group, but comparable at 36 months. Developmental plateau and regression occurred in some children with ASD, regardless of timing of ASD diagnosis. Findings indicate a preclinical phase of varying duration for ASD. (Contains 1 figure and 2 tables.)