This study tested the effects of a parent-mediated intervention on parental responsiveness with their toddlers at high risk for an autism spectrum disorder (ASD).Participants included caregivers and their 66 toddlers at high risk for ASD. Caregivers were randomized to 12 sessions of an individualized parent education intervention aimed at improving parental responsiveness or to a monitoring control group involving 4 sessions of behavioral support. Parental responsiveness and child outcomes were measured at three time points: at beginning and end of the 3-month treatment and at 12-months post-study entry. Parental responsiveness improved significantly in the treatment group but not the control group. However, parental responsiveness was not fully maintained at follow up. There were no treatment effects on child outcomes of joint attention or language. Children in both groups made significant developmental gains in cognition and language skills over one year. These results support parental responsiveness as an important intervention target given its general association with child outcomes in the extant literature; however, additional supports are likely needed to fully maintain the treatment effect and to affect child outcomes.