The relation between early joint attention (in which a child coordinates attention between another person and an object or event) and later language and social outcomes was examined in younger siblings of children with autism spectrum disorder (Sibs-ASD) and younger siblings of children with typical development (Sibs-TD). Initial levels of joint attention (at a mean age of 15 months) as well as growth in levels of joint attention (between 15 months and 34 months) were used as potential predictors of outcomes at age 5. The results revealed that initial levels of initiating joint attention (IJA) were associated with language skills at outcome. In addition, growth of responding to joint attention (RJA) was associated with social skills at age 5. These patterns of associations were not significantly different between the Sibs-TD and Sibs-ASD groups. Although the Sibs-ASD group had lower joint attention scores than the Sibs-TD group at younger ages, significant group differences were not found for most measures at age 5.