BackgroundOur aim was to compare MI-oriented versus CBT-oriented adjunctive treatments to test whether an MI approach is superior in terms of improving therapeutic alliance and engagement among individuals with an eating disorder. The current study was a pilot randomized controlled trial with random allocation to either MI-oriented or a CBT-oriented adjunctive treatment group completed concurrently with a hospital-based group program for adults. Both adjunctive treatment conditions consisted of three individual therapy sessions and a self-help manual.MethodsSixty-five outpatients receiving hospital treatment for a diagnosed eating disorder were randomly assigned to a treatment group. Measures of working therapeutic alliance, engagement, treatment completion, and clinical impairment were completed at preadmission, mid-treatment, and at the end of treatment.ResultsWorking alliance increased equivalently in both conditions over time in treatment. Similarly, there were no differences between conditions in terms of engagement. Regardless of therapy orientation, greater use of the self-help manual predicted lowered eating disorder risk; stronger patient ratings of therapeutic alliance predicted decreased feelings of both ineffectiveness and interpersonal problems.ConclusionThis pilot RCT provides further evidence that both alliance and engagement are important for treatment of an eating disorder; however, there was no clear advantage of MI over CBT as an adjunctive treatment approach to improving alliance or engagement.Trial registration: ClinicalTrials.gov ID #NCT03643445 (proactive registration).