A randomized controlled trial was conducted to evaluate effects of the Joint Attention Mediated Learning (JAML) intervention. Toddlers with autism spectrum disorders (ASD) aged 16-30 months (n = 144) were randomized to intervention and community control conditions. Parents, who participated in 32 weekly home-based sessions, followed a mediated learning process to target preverbal social communication outcomes (social visual synchrony, reciprocity, and responding and initiating forms of joint attention) throughout daily interactions. The analysis found post-intervention effects for all outcomes, with all except initiating joint attention sustaining 6 months post-intervention. Findings support the value of very early intervention targeting explicitly social functions of preverbal communication and of promoting active engagement in the learning process for both toddlers and parents.