This review of the descriptive literature on borderline patients indicates that accounts of such patients vary depending upon who is describing them, in what context, how the samples are selected, and what data are collected. The authors identify six features that provide a rational means for diagnosing borderline patients during an initial interview: the presence of intense affect, usually depressive or hostile; a history of impulsive behavior; a certain social adaptiveness; brief psychotic experiences; loose thinking in unstructured situations; and relationships that vacillate between transient superficiality and intense dependency. Reliable identification of these patients will permit better treatment planning and clinical research.