Background: Psychotherapy is an effective treatment for generalized anxiety disorder in comparison to no-treatment controls. Instead of creating more and more new overall treatment-packets within a medical meta-model, a complementary approach to investigate clinical research designs may lie into the understanding of already effective psychotherapies. Treatment manuals and protocols allow a relatively high degree of freedom of therapists' behaviors on how to implement the overall treatment manuals. There is a lack of systematical knowledge of how therapists have to customize these overall protocols. The present design experimentally examines 3 types of conducting a 15 session time-limited cognitive-behavioral therapy (CBT) protocol and its relation to the therapists' protocol adherence and treatment efficacy.