Background: Despite the notion that randomized controlled trials are regarded as the gold standard in psychotherapy research, questions about their generalizability have been raised. This paper focuses on the differences between participants and eligible nonparticipants of a randomized controlled trial for patients with borderline personality disorder (BPD). Sampling and Methods: One hundred forty-two patients were screened, and 122 were found eligible for study participation. Out of these, 64 patients (52.5%) gave informed consent and were included in the study. Results: The 58 eligible nonparticipants showed a lower level of functioning (global assessment of functioning score), had a history of more outpatient treatment attempts and were living alone more often. Regarding acute symptoms and severity of BPD as indexed by suicide attempts, inpatient treatments, substance abuse and history of trauma, no differences between the groups could be detected. Moreover, participants showed significantly more eating disorders, whereas nonparticipants presented more affective and anxiety disorders. Conclusions: The results indicate that lower psychosocial functioning and comorbid affective and anxiety disorders decrease BPD patients' willingness to participate in an RCT. Copyright (C) 2010 S. Karger AG, Basel