There have been calls for the creation of a non-suicidal self-injury (NSSI) disorder, and a recent DSM-5 work group has delineated potential diagnostic criteria for the proposed disorder. Preliminary research comparing NSSI disorder to Axis I disorders and its closest diagnostic ‘neighbor’, borderline personality disorder (BPD), suggests that impairment at intake, patient history of associated features and diagnostic co-occurrence rates set NSSI disorder apart from existing DSM-IV-TR disorders. However, few studies have examined the treatment course of NSSI disorder relative to other disorders. The purpose of this study was to distinguish potential treatment outcome differences between a potential NSSI disorder, BPD and Axis I. Archival data (N = 571), which included ratings of functioning at therapy intake and termination, were analysed to determine between-group differences. Although no significant differences in the number of months in therapy or number of sessions attended were found, findings suggest that the NSSI group made strong gains in therapy, as evidenced by improvements in clinician ratings of functional impairment from intake to termination. These findings suggest that NSSI disorder may have a positive prognosis with treatment. Copyright © 2012 John Wiley & Sons, Ltd.