This study follows 40 children who were participants in a randomized controlled early intervention trial (Kasari etal.) from early childhood (2-5 years of age) to elementary school age (8-10 years). To fully utilize the available longitudinal data, the general linear mixed model was the primary analytical approach. The growth trajectories of joint attention skills (pointing, coordinated joint looking, and showing) and expressive language outcomes in these children were estimated based on five time points during the measurement period. The children were grouped by diagnosis at the last follow-up (autism, autism spectrum disorder (ASD), no diagnosis) and by their original treatment group assignment (joint attention, symbolic play, control), and differences between these groups were evaluated. Results showed that joint attention skills of coordinated joint looking and showing increased over time, and pointing to share interest increased over the first year measured and decreased thereafter. These trajectories were influenced by both original treatment assignment and diagnostic status at follow-up. In addition, a cross-lagged panel analysis revealed a causal relationship between early pointing and later language development. This study highlights the longitudinal and developmental importance of measures of early core deficits in autism, and suggests that both treatment and ASD symptomatology may influence growth in these skills over time. Autism Res 2014, 7: 207-215. © 2014 International Society for Autism Research, Wiley Periodicals, Inc.