Synchronous positive affect (SPA) is a key element of parent-child interaction quality which is related to favorable developmental outcomes. Children with autism spectrum disorder (ASD) and their parents tend to show less SPA compared to other populations. The current study explored changes in SPA made by parents and their children with ASD following the Preschool-Based Early Start Denver Model (PB-ESDM) intervention. Thirty children receiving PB-ESDM and 23 receiving treatment-as-usual (TAU) were assessed pre- and post- intervention using microanalysis of video-recorded parent-child interactions, in which SPA was quantified. Results showed a significant increase in SPA among children receiving PB-ESDM who had lower pre-treatment adaptive functioning. These findings suggest that SPA may serve as a sensitive treatment outcome measure for children with poorer adaptive functioning, who often struggle to show significant changes on standardized measures. The study's modest sample and non-randomized design are noted as limitations.