Over the last decade, considerable progress has been made to increase our understanding of the pathophysiology of autism spectrum disorder (ASD), but limited progress has been made in identifying effective interventions. The goal of this investigation is to build on our preliminary findings and conduct a randomized controlled 24-week pilot trial to examine the efficacy of a pivotal response treatment package (PRT-P) in improving language expression in young children with ASD. Pivotal response treatment (PRT) is a naturalistic behavioral intervention based on the principles of applied behavior analysis. While some evidence has emerged suggesting the effectiveness of PRT, no randomized well-controlled trials have been published to date examining the efficacy of this intervention in ASD. The proposed treatment model will focus on teaching parents PRT strategies in combination with in-home therapy for 24 weeks and will be compared to families in a delayed treatment group (DTG). To achieve these goals, 48 children with autism (age range: 2-5.11 years) with severe communication deficits will be recruited to participate in this study and will be stratified according to gender. Twenty-four children will be randomized to the PRT-P and 24 to the DTG. The PRT-P will consist of a combination of parent training sessions and in-home therapy for a total of 10 weekly treatment hours for the first 12 weeks, and 5 hours per week for the following 3 months. Standardized and video-taped assessments will be conducted at baseline, week 6, 12, and 24 and will be rated by a blind investigator. In addition, full-day audio recordings of the child's natural language environment will be analyzed using the Language ENvironment Analysis System (LENA), which involves digital language recording and software analysis. Finally, a social attention and word-learning eye tracking task will be used to monitor the effectiveness of treatment on social and communication deficits. This research will show that parents of children with autism can learn pivotal response training procedures and their children will benefit with improved language skills. Additionally, we are interested in exploring whether response to treatment will vary according to any clinical or cognitive factors at baseline (moderators). 1