Objective: Parent-child interaction therapy (PCIT) has been found to reduce future child abuse reports among physically abusive parents. Reductions in observed negative parenting behaviors mediated this benefit. The current study examined session-by-session interaction sequences in order to identify when during treatment these changes occur and how much the trajectory varies from case-to-case. Method: Session-by-session parent-child interaction sequences, using the Dyadic Parent-Child Interaction Coding System-II (DPICS-II) categories, were coded for 22 child welfare involved parent-child dyads undergoing PCIT for child physical abuse. A total 5,436 interactions across PCIT were coded and analyzed using growth curve analysis. Results: At pre-treatment baseline, negative and positive parental responses were about equally likely to follow a child positive behavior. This pattern changed rapidly during PCIT, with rapid increases in positive parental responses and decreases in negative parental responses to appropriate child behavior. A quadratic growth pattern accounted for 70% of observed variance and virtually all change occurred during the first three sessions. Conclusion: Changes in observed abusive parent-abused child interaction patterns can occur early in PCIT, a parenting intervention that involves direct coaching and practice of skills. These benefits sustained throughout treatment. Practice implication: Prior to receiving behavioral parent training (PCIT), parents who have physically abused their children failed to match their parental response to their children's behavior. This pattern of interaction improved rapidly and substantially during the first three sessions of PCIT. The changes in the patterns of interaction also remained relatively stable for the remainder of treatment while parents continued to practice positive parental responses as well as began practicing effective discipline techniques. This suggests that use of immediate parent feedback through coaching, explicit directions to parents in how to respond to child behavior, and customization of the application of skills to the problems that arise in session are important components to effective parenting programs with physically abusive parents. Targeting these behaviors with PCIT has been found to reduce rates of recidivism, further supporting clinical application of PCIT in these cases. (Contains 2 tables and 3 figures.)