The cases of two pairs of female monozygotic twins concordant for anorexia nervosa are considered in order to explore the interrelationship between the psychodynamics of the twin relationship and the psychopathology of anorexia. It is suggested that twins face a dual task of separation-individuation - from mother and from the other twin - and that separation-individuation between monozygotic twins presents particular difficulties. Anorexia nervosa may represent an attempt to avoid separation by means of a retreat to a pre-adolescent state in which differences, envy and rivalry can be denied.