There is a lack of information in the literature on patients with borderline personality disorder (BPD) who do not respond sufficiently to the standard one-year dialectical behavior therapy (DBT). In this naturalistic, observational study, we compared 43 patients completing and responding to one year of DBT (m=34.5 years, 83.7% women; first-year responders, R1) with 12 patients (m=34.4 years, 91.7% women) with BPD who completed but did not respond clinically to the standard one-year DBT (first-year non-responders, NR1). We also followed the NR1's progress in treatment during a second consecutive year of DBT. Despite a lack of difference in clinical characteristics at baseline, progress of R1 and NR1 patients differed substantially and significantly during the first year of DBT. Over two years, the NR1 patients significantly improved on all outcome measures. However, most of the changes became significant only after 18 months of therapy. The proportion of NR1 patients attaining a Global Assessment of Functioning score>60 after their second year of treatment was close to that of R1 patients (58.3% and 62.8%, respectively). Despite improvements in BPD symptomatology and an increased use of DBT skills, rates of reliable clinical change indicated that these NR1 patients remained vulnerable to depression and emotion regulation difficulties.