["Autism Research, EarlyView. ", "\nABSTRACT\nDisruptive behaviors (non‐compliance, aggression) are common in autistic children. Parent–Child Interaction Therapy (PCIT) is a caregiver‐mediated intervention utilizing in vivo caregiver coaching that reduces disruptive behaviors and has been shown to be effective for young autistic children. This study sought to identify child and caregiver factors that relate to session‐level progress among autistic children and their caregivers. Participants included 98 autistic children aged 2–8 years (91% male) who received PCIT in a university‐affiliated clinic. Session‐level change was examined using iterative multilevel modeling to examine change in child disruptive behaviors, as measured on the Eyberg Child Behavior Inventory (ECBI), caregiver positive attending skills (Do‐skills), and caregiver negative statements (Avoid‐skills). Optimal fitting trajectories revealed quadratic change in ECBI scores, Do‐skills, and Avoid‐skills. The effectiveness of PCIT did not differ based on participants' level of autistic traits or caregiver demographics. Improvements in child disruptive behavior were associated with child adaptability and parenting stress pre‐treatment; higher adaptability and lower stress were related to faster declines in child disruptive behavior. Caregivers exhibited slower rates of change in Do‐skills when they had lower rates of homework completion, and when their child had lower social responsiveness, and challenges with adaptability and externalizing problems. These findings may help clinicians tailor what factors to emphasize and monitor over the course of PCIT.\n"]