A review of high intensity, high dose mentalization-based inpatient psychiatric treatment indicated large effect-size reductions in symptoms of depression, anxiety, somatization, and improving emotion-regulation functioning (Allen et al., 2017). This study examined the impact of pathological personality traits has on baseline symptoms and functioning, as well as their impact on the longitudinal course in a large cohort of adult inpatient psychiatric sample (N = 804). The Personality Inventory for DSM-5 (PID-5; Krueger et al., 2012) was used to assess trait domains impact on longitudinal outcomes (anxiety, depression, somatic symptoms, and functional impairment) using hierarchical repeated measures modeling. Results indicate Negative Affectivity and Detachment were related to higher admission severity in all four outcome domains. Psychoticism was related to somatic symptoms, while Antagonism and Disinhibition were related to functional impairment. Paradoxically, when symptoms were plotted over 2-week intervals during hospitalization, patients with higher admission PID-5 trait scores exhibited greater improvement over time. The PID-5 appears to contribute to prediction of treatment outcome response above and beyond demographic and burden of illness. Importantly, the findings add to a growing body of literature indicating that impairments in personality traits do not preclude positive treatment response, particularly when treatments target pathological personality features. (PsycInfo Database Record (c) 2022 APA, all rights reserved).