Positive emotional engagement develops in the context of face-to-face interactions during the first 6 months of life. Deficits in emotional engagement are characteristic of autism spectrum disorder (ASD) and may characterize the younger siblings of children with ASD (high-risk siblings). High-risk siblings are likely to exhibit a broad range of positive emotional engagement that may or may not be associated with ASD outcomes. We examined positive emotional engagement (i.e., smiling rate and contingent responsiveness to the partner's smile) during the infant-parent interaction episodes of the face-to-face/still face protocol at 6 months of age. The sample included 43 high-risk infant siblings, 11 of whom went on to an ASD diagnosis, and 25 low-risk siblings with no family history of ASD. Low-risk siblings and high-risk siblings without ASD showed the typical "still-face effect" (i.e., decreases in smiling rate after period of parental nonresponsiveness), but high-risk siblings with later ASD outcomes did not show this decrease. Although high-risk siblings without an ASD diagnosis were less likely to respond to their parents' smiles than were low-risk siblings, the children with eventual ASD did not differ from the other groups in contingent responsiveness. Findings suggest that subtle differences in positive emotional engagement are present in the early development of high-risk siblings but are not consistently associated with ASD outcomes.