Despite evidence for the efficacy of several treatments for deliberate self-harm (DSH) within borderline personality disorder (BPD), predictors of response to these treatments remain unknown. This study examined baseline demographic, clinical, and diagnostic predictors of treatment response to an adjunctive emotion regulation group therapy (ERGT) for DSH among women with BPD. A recent RCT provided evidence for the efficacy of this ERGT (relative to a treatment-as-usual only waitlist condition). Participants in this study include the full intent-to-treat sample who began ERGT (across treatment and waitlist conditions; n = 51). Baseline diagnostic and clinical data were collected at the initial assessment, and outcome measures of DSH and self-destructive behaviors, emotion dysregulation/avoidance, and BPD symptoms (among others) were administered at pretreatment, posttreatment, and 3- and 9-months posttreatment. Notably, both demographic variables and characteristics of participants' ongoing therapy in the community had minimal impact on treatment response. However, several indicators of greater severity in domains relevant to this ERGT (i.e., baseline emotion dysregulation and BPD criteria, lifetime and recent DSH, and past-year hospitalization and suicide attempts) predicted better responses during treatment and follow-up across the primary targets of treatment. Likewise, several co-occurring disorders (i.e., social phobia, panic disorder, and a cluster B personality disorder) predicted greater improvements in BPD symptoms during treatment or follow-up. Finally, although co-occurring generalized anxiety disorder, posttraumatic stress disorder, and cluster A and C personality disorders were associated with poorer treatment response during follow-up, most of these effects reflected a lack of continued improvements during this period (vs. worsening of symptoms).