Even the most effective treatments for bulimia nervosa and purging disorder have high rates of nonremission and relapse. As such, improving treatment efficacy is an important research priority in this area. Research has consistently demonstrated that rapid response – defined as substantial improvements in key eating disorder behaviours (e.g., binge eating, vomiting, dietary restriction) during the initial weeks of cognitive behavioural therapy (CBT) – is the strongest and most robust predictor of good outcomes at end-of-treatment and in follow-up (Vall & Wade, 2015). Further, research has failed to identify pre-treatment demographic or clinical variables that account for this relationship, suggesting that rapid response is due to elements of CBT itself. This study aimed to demonstrate that rapid response can be clinically facilitated. A four-session CBT intervention focused on encouraging rapid response was compared to a matched-intensity motivational interviewing intervention, both adjunctive to intensive treatment in a randomized controlled trial. The CBT intervention included psychoeducation about rapid response, a focus on goal-setting, and use of behavioural skills for making concrete changes. Forty-four women with bulimia nervosa or purging disorder participated in the study. There were no baseline differences between groups on any demographic or clinical variables. Intent-to-treat results showed that compared to those who received motivational interviewing, participants who received CBT were significantly more likely to make a rapid response to day hospital treatment, and had fewer total eating disorder behaviours and more normalized eating during the first 4 weeks of day hospital treatment. Additionally, between baseline and day hospital end-of-treatment, participants who received CBT made significantly greater improvements on overvaluation of weight and shape and difficulties with emotion regulation. These findings indicate that rapid response to intensive treatment can be clinically facilitated using an adjunctive intervention focused on encouraging rapid and substantial change. These findings also suggest that rapid response may be related to improved outcome via improvements in overvaluation of weight and shape or emotion regulation. This study provides support for theoretical contentions that rapid response is due to CBT-related factors, and provides the framework for future research investigating rapid response as a causal mechanism of good outcome for eating disorders.