Language development in typically developing children (TD) has traditionally been investigated in relation to environmental factors, while language in children with autism spectrum disorder (ASD) has primarily been related to child-based factors. We employ a longitudinal corpus of 32 preschoolers with ASD and 35 linguistically matched TD peers recorded over 6 visits (ranging between 2 and 5?years of age) to investigate the relative importance of child-based and environmental factors in language development for both populations. We also investigate the reciprocal interaction between children’s response to parents’ input, and parents’ response to children’s production. We report six major findings. (1) Children’s production of word types, tokens, and MLU increased across visits, and were predicted by their Expressive Language (EL) (positively) and diagnosis (negatively) from Visit 1. (2) Parents’ production also increased across visits, and was predicted by their child’s nonverbal cognition (positively) and diagnosis (negatively) from Visit 1. (3) At all visits and across groups, children and parents matched each other in lexical and syntactic production; (4) Parents who produced longer MLUs during a given visit had children who produced more word types and tokens, and had longer MLUs, at the subsequent visit. (5) When both child EL at Visit 1 and parent MLU were included in the model, both contributed significantly to future child language; however, EL accounted for a greater proportion of the variance. (6) Finally, children’s speech significantly predicted parent speech at the next visit. Taken together, these results draw more attention to the importance of child-based factors in the early language development of TD children, and to the importance of parental language factors in the early language development of children with ASD.