Borderline personality disorder (BPD) is a diagnosis characterized by intense and labile emotion; dialectical behavior therapy, a common treatment for BPD, aims to reduce the intensity and lability of clients' emotion through multiple methods, some of which occur in the therapy session, with the expectation that changes will generalize to the rest of clients' lives. However, little research has examined how BPD clients' affect presents and varies in session or whether affect in session reflects patients' patterns of affect outside of treatment. This study had 2 aims: (a) to explore changes in clients' positive and negative affect in therapy, and (b) to assess if the severity of client psychopathology relates to affect in treatment. Positive and negative affect ratings were collected from clients (N = 73) at the start and end of every individual therapy session (total sessions = 1,474). Hierarchical linear modeling and linear regression were used to examine patterns of affect and assess the relationship between affect and severity. Results indicated that positive affect increased while negative affect decreased between the start and end of sessions, with the same pattern of change in presession affect from week to week. In addition, increased BPD severity was associated with lower presession positive affect ratings and higher negative affect ratings. Further exploration is needed to assess which dialectical behavior therapy treatment processes contribute to changes in in-session affect and how in-session affect relates to treatment outcomes. (PsycINFO Database Record (c) 2019 APA, all rights reserved).