Unstructured mother-toddler interactions were examined in 18-month-old high- and low-risk children subsequently diagnosed (n = 12) or not diagnosed (n = 21) with autism spectrum disorders (ASD) at 36 months. Differences in maternal sensitivity were not found as a function of emergent ASD status. A differential-susceptibility moderation model of child risk guided investigations linking maternal sensitivity to child behavior and language growth. Group status moderated the relation between sensitivity and concurrent child behavior problems, with a positive association present for children with emergent ASD. Maternal sensitivity at 18 months predicted expressive language growth from age 2 to 3 years among children with emergent ASD only. Findings underscore the importance of understanding parent-child interaction during this key period in the development of autism symptomatology.