Siever and Gunderson (1979) have questioned the decision to separate Schizotypal Personality Disorder from Borderline Personality Disorder in DSM-III. The justification for this separation rests not on genetic evidence, but rather on the relative independence of the behavioral characteristics of two dimensions that up to now have both been referred to with the appellation “borderline.” We believe that this separation provides the tools with which investigators may usefully study the interaction of genetic and environmental factors as they relate to personality and the major psychiatric disorders. The benefits of this separation are already apparent in that research investigators are now using two terms to describe different phenomena, when previously they were using the single term borderline. Proposed diagnostic criteria for Schizotypal Personality Disorder and Borderline Personality Disorder are appended.