Background <p>Emotion Regulation Therapy (ERT) integrates components of traditional cognitive behavioral therapy (CBT) with mindfulness/acceptance practices and experiential techniques through an affective science framework to modify perseverative negative thinking (e.g., worry, rumination) by targeting proposed motivational, regulatory, and behavioral mechanisms. As with other CBTs, case conceptualization helps therapists track patients’ progress and informs personalization of treatment to match patients’ needs. However, whether convergence between therapist case conceptualizations and patient-ratings on treatment mechanisms leads to meaningful changes in clinical outcomes has yet to be empirically explored.</p> Method <p>In this study, 98 adults (75% female, 52% White, 75% non-Hispanic), with primary generalized anxiety disorder (GAD) or co-primary GAD and major depressive disorder, completed 16–20 sessions of ERT. Therapists (<i>N</i> = 41) and patients rated patient progress on treatment mechanisms at pre-treatment, mid-treatment, and post-treatment using case conceptualizations and self-report scales, respectively. Specifically, we examined attentional regulation (AR; the ability to broaden and sustain attention), metacognitive regulation (MR; the ability to see thoughts, feelings, and memories with healthy distance/perspective in time and to speak back to self-critical, ruminative, and worried thoughts with courage and compassion), and clinical outcomes (anxiety/worry symptoms, depression symptoms, and disability/functional impairment).</p> Results <p>Multilevel modeling (MLM) with restricted maximum likelihood method (REML) was first used to explore change in therapist and patient ratings of AR and MR. Growth models revealed that both therapist- and patient-ratings of AR and MR increased over treatment. MLM was then further used to explore covariation between therapist- and patient-ratings of patient progress, which revealed that ratings of AR (but not MR) positively covaried throughout treatment. Next, we examined whether the relationship between therapist- and patient-ratings of AR and MR changed over time, which revealed a trending (<i>p</i> =.09) time x patient-rating interaction on therapist ratings of AR. Exploratory follow-up tests to probe this interaction revealed positive covariance from pre- to mid-treatment <i>and</i> from mid- to post-treatment, both with medium effect sizes. Finally, using stepwise linear regressions, we explored whether residuals extracted from this change in AR covariance model predicted posttreatment outcomes, controlling for pre-treatment levels. Model residuals significantly predicted all clinical outcomes, accounting for 2–3% of the variance.</p> Conclusions <p>This study represents a first attempt at evaluating whether convergence between therapist case conceptualization and patient ratings of progress on targeted treatment mechanisms predicts clinical outcomes. Findings are preliminary and limitations include sample demographics, alignment of measurement timepoints, and use of different scales for therapists and patients. Future research should explore whether patient-therapist convergence can be used as a just-in-time shared decision-making tool to guide personalization of the dosing of treatment components to match patients’ individual needs.</p>