The Separation Individuation Inventory (SII; Christenson and Wilson,J. Nerv. Ment. Dis., 173:561–565, 1985) was designed to assess adult psychopathology construed in terms of the concepts of separation and individuation (Mahler, Pine, and Bergman,The Psychological Birth of the Human Infant: Symbiosis and Individuation, 1975). Its reported use is confined to the original authors' paper, which compared scores of 20 subjects with borderline personality disorder with the scores of staff control subjects. We report the association of the SII with two self-report measures of borderline personality diagnosis and phenomenology. Our British samples, from the staff and residents in a therapeutic community for severely personality-disordered clients, thus complement the original American work. The SII was found to have good internal consistency, respectable screening parameters, and high correlations with indications of trait psychopathology. We suggest a slight modification to the scoring of the inventory and two main areas for additional research.