Behavioral characteristics of children with autism spectrum disorder (ASD) who respond positively to Pivotal Response Treatment (PRT) have been described previously, based on single-subject design research. The present study examined several such characteristics, as well as positive affect, as predictors of expressive language (EL) gains in a representative sample of preschoolers with ASD (n = 57) enrolled in a PRT-based community early intervention program. Children's cognitive ability, positive affect, and levels of appropriate toy contact measured at the start of intervention each contributed significantly to the prediction of EL outcomes. Together these variables accounted for 40% of the total outcome variance. In addition, a profile of increased EL ability, positive affect and appropriate toy contact, and decreased social avoidance and stereotyped and repetitive vocalizations was associated with greater gains during intervention. Results are discussed in relation to their implications for understanding both the variable treatment response documented in children with ASD and how to tailor treatment to optimize individual benefit. Autism Res 2018, 11: 153-165. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.