Response to joint attention (RJA) is impaired in preschoolers with autism spectrum disorder (ASD) and is pivotal to social and communication development. Response to joint attention was examined at 14 and 24 months in 51 children at high risk for autism (siblings of children with autism). Outcome groups at age 3 years included ASD (n = 16), broader autism phenotype (n = 8), and non-broader autism phenotype (n = 27). The ASD group made minimal improvement in RJA between 14 and 24 months, but stability of RJA across tasks increased for all three groups. Significantly, lower RJA was observed for the ASD group at 24 months. Response to joint attention performance at 14 months predicted ASD outcome. Response to joint attention is an important screening and early intervention target.