The question of developing a therapeutic environment was prominent throughout Winnicott’s career. We present an unpublished clinical case from Winnicott, based on previously unknown correspondence with Anna Freud. We begin by providing historical context for Winnicott’s views on the psychoanalytic theory proposed by the two pioneers of child psychoanalysis, Anna Freud and Melanie Klein. We present nine unpublished letters, mostly from Winnicott to Freud, which allow us to reconstruct important aspects of the psychoanalytic treatment of a young girl with anorexia. Hypotheses about the patient’s unconscious world are connected with Winnicott’s involvement in establishing a therapeutic environment that includes a range of practitioners.