This study explores the relationship between psychotherapists' validation interventions and patients' metacognitive responses at the beginning of treatment of borderline personality disorder (BPD). A model of BPD based on disorganized attachment provides the hypothesis that, before patients' internal working model of attachment has been corrected within the therapeutic relationship, therapist interventions that are likely to activate patients' attachment system are also likely to induce temporary disorganization of patients' metacognitive functions. Any validation intervention implies that therapists openly display an understanding and accepting attitude when they comment on patients' reported experiences and is, therefore, likely to activate the patients' attachment system. Linehan's (1993) manual of dialectic-behavioral therapy (DBT) was used as a guideline to assess validation interventions adopted by therapists. The transcripts of the second individual session in the psychotherapy of 19 consecutive patients were analyzed. Checklists based on the DBT manual were used to identify therapists' validating, supportive, and neutral interventions. The Metacognitive Assessment Scale was used to assess changes in specific aspects of patients' metacognitive processes during therapeutic dialogues. Following validation interventions, patients' responses revealed significantly higher rates of temporary metacognitive failure in comparison to the responses solicited by neutral intervention.