Diversity of key consonants used in communication (DKCC) is a value-added predictor of expressive language growth in initially preverbal children with autism spectrum disorder (ASD). Studying the predictors of DKCC growth in young children with ASD might inform treatment of this under-studied aspect of prelinguistic development. Eighty-seven initially preverbal preschoolers with ASD and their parents were observed at five measurement periods. In this longitudinal correlational investigation, we found that child intentional communication acts and parent linguistic responses to child leads predicted DKCC growth, after controlling for two other predictors and two background variables. As predicted, receptive vocabulary mediated the association between the value-added predictors and endpoint DKCC.