Research on the expression of positive affect in young children with Autism Spectrum Disorder (ASD) suggests that differences in this domain emerge late in the first year or early in the second year. However, many previous studies in this area employed retrospective research methods and global rating schemes. In the current study, the expression of positive affect was examined prospectively at ages 6, 12, and 18 months in three groups: infant siblings with ASD, infant siblings without ASD, and low-risk comparison infants. Infant siblings were the younger brothers or sisters of children diagnosed with ASD and, therefore, had a higher familial risk of ASD. The frequency and duration of smiles were coded from video excerpts from the Autism Observation Scale for Infants (Bryson, Zwaigenbaum, McDermott, Rombough, and Brian 2008), a standardized, play-based assessment of early signs of ASD. Results indicated that at 12 months, infant siblings with ASD had a lower rate of smiling than the other two groups. At 18 months, infant siblings with ASD continued to display a lower rate of smiling than infant siblings without ASD, but not comparison infants. Overall, these results indicate that infant siblings with ASD demonstrate less positive affect than infant siblings without ASD and low-risk comparison infants at 12 months. This suggests that reduced smiling may be an informative behavioural risk marker for ASD by children's first birthdays and may have implications for our understanding of atypical social development in children with ASD.