Longitudinal research has demonstrated that responsive parental behaviors reliably predict subsequent language gains in children with autism spectrum disorder. To investigate the underlying causal mechanisms, we conducted a randomized clinical trial of an experimental intervention (Focused Playtime Intervention, FPI) that aims to enhance responsive parental communication (N = 70). Results showed a significant treatment effect of FPI on responsive parental behaviors. Findings also revealed a conditional effect of FPI on children's expressive language outcomes at 12-month follow up, suggesting that children with baseline language skills below 12 months (n = 24) are most likely to benefit from FPI. Parents of children with more advanced language skills may require intervention strategies that go beyond FPI's focus on responsive communication.