The concepts of ‘borderline’ and ‘borderline schizophrenia’ remain a confusing area in psychiatry. In its functional use, the word ‘borderline’ refers to a hypothesized level of functioning on a continuum which extends from normality to psychosis, e.g. borderline personality organization. In the phenomenological use ‘borderline’ qualifies a psychiatric diagnosis, referring to a milder form of the disease process, e.g. borderline schizophrenia. In this paper we attempted to define and validate the concept of borderline schizophrenia using case records from the Danish Adoption study with this diagnosis and rating the cases on a new instrument: the Symptom Schedule for the Diagnosis of Borderline Schizophrenia, found reliable, and discriminating cases of borderline schizophrenia from cases of neurosis and personality disorder.