Objective: Criterion A serves as the fundamental diagnostic criterion of the Alternative Model of Personality Disorders in section III of the Diagnostic and Statistical Manual 5. Consisting of a self- and an interpersonal dimension, it defines the construct of personality functioning as a general and dimensional factor of personality disorders. This study aimed to explore criterion A along with well-established treatment dropout predictors, e.g., sociodemographic factors, personality disorder diagnosis, symptom severity, and the therapeutic alliance. Methods: The sample consisted of 132 patients diagnosed with personality disorder in a psychotherapeutic inpatient treatment. Cox proportional hazard regression models and a lasso model were applied. Results: 28% of the sample prematurely discontinued treatment. The risk for dropout was 2.3 times higher for patients with high impairments in self-functioning as assessed with criterion A. Moreover, a positive therapist-rated therapeutic alliance was associated with a lower dropout risk. Conclusion: The study suggests criterion A is a useful clinical indicator by identifying patients with personality disorder with a higher risk for dropout. An individualized therapeutic approach for such patients might be required.