Case formulation is a central tool for psychotherapists, which helps them tailor psychotherapy to the individual patient, particularly for treatments for complex and multilayered clinical problems, such as personality disorders (Kramer, 2019). Case formulation methodologies are still underutilized in psychotherapy research in the prediction of therapy processes. The present study included N = 60 patients with borderline personality disorder undergoing a brief treatment using an individualized treatment component (n = 31), as compared with a standard brief treatment (n = 29; Kramer et al., 2014). For each patient (in both groups as post hoc analysis based on videos), we performed a Plan analysis case formulation (Caspar, 2019): the idiographic information from the formulation was translated into quantitative scores (on a Likert-type scale) assessing patient's interactional agreeableness (vs. antagonism; Zufferey et al., 2019). We modeled the session-by-session predictions of the progression of the therapeutic alliance-rated by the patient and the therapist-over the course of treatment, as a function of interactional agreeableness, the individualization of treatment, as well as their interaction with the session number. Patients with high levels of agreeableness have a significant increase in their alliance assessment over time. Treatment based on the case formulation predicted session-by-session increase of the therapeutic alliance as rated by the therapists. This study was the first to explore intra- and interindividual dynamics of the therapeutic alliance in relationship with idiographic information extracted from case formulations. The results may help understand relationship struggles at the beginning of therapy for complex clinical problems, such as borderline personality disorder. (PsycInfo Database Record (c) 2023 APA, all rights reserved).