OBJECTIVE: This article describes the clinical and theoretical significance of intolerance of aloneness for patients with borderline personality disorder. It is intended to make their treatment more effective and less burdensome. METHOD: Clinical observations from the author's more than 9,000 hours of psychotherapeutic work and 500 psychotherapy consultations with borderline patients are synthesized with findings of relevant empirical studies and attachment theory. RESULTS: Intolerance of aloneness is a deficit that is associated with the borderline patient's typical clinging and attention-seeking or detached forms of attachment. Suggestions are given for ways in which clinicians can respond to these dysfunctional attachment behaviors to diminish the patient's feared aloneness without encouraging unnecessary regressions. A framework for understanding the long-term attachment processes required to correct this deficit is offered. CONCLUSIONS: Intolerance of aloneness is a core deficit in borderline patients that can become less handicapping with reliable, but not excessive, responsiveness of the therapist.