Hermeneutic Single-case Efficacy Design (HSCED) was applied to a good-outcome case in which an individual presenting with social anxiety (SA) received 13 sessions of nondirective Person-centered therapy. A Rich Case Record of qualitative and quantitative data gathered over the course of therapy was constructed, and provided the basis for affirmative and skeptic arguments. A panel of three judges returned verdicts on the client’s therapeutic process and its outcome. Judges concluded unanimously that the client changed substantially over the course of therapy and that therapy contributed substantially to client change (80% probability). The judges emphasized the contribution of the therapeutic relationship, especially unconditional positive regard. Judges identified the therapist’s non-directive stance, openness and empathy, and reflection and summarizing in session as additional qualitative mediators of change. Judges identified the client’s own readiness for change, understanding of the therapeutic process and ability to use the research processes to reflect on therapy as moderators of change. We discuss the implications for Person-Centered-Experiential therapies as a credible treatment for SA, and for the further development of HSCED as an approach to evaluating psychotherapy.